AGRICULTURAL  GEOLOGY 


CAMBRIDGE   GEOLOGICAL  SERIES 


AGRICULTURAL    GEOLOGY 


CAMBRIDGE    UNIVERSITY    PRESS 

C.   F.   CLAY,  MANAGER 

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fcombag,  (Calcutta  anH  Jflafcras  :   MACMILLAN  AND   CO.,    LTD. 
Toronto:  J.    M.   DENT  AND   SONS,   LTD. 
THE  MARUZEN-KABUSHIKI-KAISHA 


All  rights  reserved 


AGRICULTURAL  GEOLOGY 


BY 


R.    H.    RASTALL,    M.A. 

Late  Fellow  of  Christ's  College  and  Demonstrator  of  Geology 
in  the  University  of  Cambridge 


Cambridge : 

at    the    University    Press 
1916 


Q 


PREFACE 

OF  late  years  a  large  and  increasing  amount  of  attention 
has  been  paid  to  the  practical  applications  of  geological 
science.  The  present  book  has  been  written  with  the  object 
of  supplying  a  concise  account  of  those  parts  of  geology  which 
are  of  direct  interest  to  the  agriculturist.  The  study  of  the 
soil  naturally  forms  the  most  important  part  of  the  book,  and 
this  has  been  treated  as  much  as  possible  from  a  purely  geo- 
logical standpoint:  the  chemical  and  physical  properties  of 
soils  are  dealt  with  exhaustively  in  many  books.  It  is  to  be 
regretted  that  considerations  of  space  did  not  allow  of  a  fuller 
discussion  of  the  modern  mechanical  methods  of  soil-analysis 
that  have  proved  so  valuable  in  the  hands  of  workers  at 
Rothamsted,  Cambridge  and  elsewhere.  The  later  chapters 
of  the  book  contain  a  summary  of  the  distribution  of  the 
rock-formations  of  the  British  Isles,  and  the  characters  of  the 
soils  yielded  by  them.  Much  difficulty  was  experienced  in 
obtaining  the  necessary  information,  which  is  still  far  from 
complete.  The  facts  had  to  be  collected  from  many  scattered 
sources,  especially  from  the  memoirs  of  the  Geological  Survey, 
the  Journal  of  the  Royal  Agricultural  Society,  county  histories 
and  geographies  and  many  other  publications.  Much  valuable 
information  was  also  derived  from  Mr  A.  D.  Hall's  Pilgrimage 
of  British  Farming.  I  must  here  express  my  gratitude  to 
those  gentlemen,  too  numerous  to  mention  individually,  who 
afforded  me  information  in  reply  to  direct  enquiries. 

For  much  help  and  encouragement  in  the  preparation  of 
the  book  I  am  indebted  to  Mr  K.  J.  J.  Mackenzie,  Reader 
in  Agriculture  at  Cambridge,  and  to  Dr  F.  H.  A.  Marshall, 

458498 


vi  PREFACE 

Fellow  and  Tutor  of  Christ's  College,  Cambridge.  The  latter 
was  so  kind  as  to  undertake  the  preparation  of  the  last  chapter 
of  the  book,  on  the  geological  history  of  farm  animals.  Per- 
mission to  reproduce  the  figures  illustrating  this  chapter  was 
kindly  given  by  Professor  J.  C.  Ewart,  Messrs  George  Allen 
and  Unwin,  Ltd.,  Messrs  Methuen  and  Co.,  and  the  Highland 
and  Agricultural  Society.  Professor  J.  C.  Ewart  and  Professor 
T.  MCK.  Hughes  also  supplied  some  information  incorporated 
in  this  chapter. 

Many  of  the  text-figures  in  the  book  were  drawn  for  me  by 
Dr  J.  E.  Marr,  to  whom  my  thanks  are  due.  Finally,  I  am 
much  indebted  to  Mr  W.  H.  Wilcockson,  who  read  all  the 
proofs  and  assisted  in  the  preparation  of  the  index. 

R.  H.  R. 
April  1916. 


CONTENTS 

CHAP.  PAGE 

I.  INTRODUCTION.     MINERALS  AND  ROCKS         .         .-         .  1 

II.  WEATHERING       .         .       .  .         .        •„         .         .    •    ,.'  38 

III.  TRANSPORT  AND  CORRASION          .         •  -      •         *         * "  60 

IV.  SEDIMENTS           .         .      '  .         .         .         ...         .  80 

V.  SUPERFICIAL  DEPOSITS         .         .         .         .         •'...'  Ill 

VI.  SOILS       .    .         .         .         .         .         .         .        \        ,.  137 

VII.  THE  GEOLOGY  OF  WATER  SUPPLY  AND  DRAINAGE     v  .  166 

VIII.  GEOLOGICAL  MAPS  AND  SECTIONS          .         .         ,         •  182 

IX.  STRATIGRAPHICAL  GEOLOGY.     INTRODUCTION                 '.  193 

X.  THE  PRECAMBRIAN  AND  LOWER  PALAEOZOIC  SYSTEMS  .  200 

XI.  THE  DEVONIAN  AND  CARBONIFEROUS  SYSTEMS      .      '   .-  216 

XII.  THE    PERMIAN    AND    TRIASSIC    SYSTEMS    ("NEW    RED 

SANDSTONE")       .         .         .         .         .         ..       .  240 

XIII.  THE  JURASSIC  SYSTEM         .         .         .         . '       ....  250 

XIV.  THE  CRETACEOUS  SYSTEM    .         ...         .  ,  _    .         .  269 
XV.  THE  TERTIARY  SYSTEMS^     .         .-         .         .         ...  283 

XVI.  THE  PLEISTOCENE  AND  RECENT  FORMATIONS       •.         .  291 

XVII.  THE  GEOLOGICAL  HISTORY  OF  THE  DOMESTIC  ANIMALS  302 

INDEX  329 


ILLUSTRATIONS 

?IG.  PAGE 

1.  Quartz  crystal  .         .  .       ,      ,  .  .  •     .         .         .         .         .6 

2.  Felspar  crystal          .         .         '.         .         ,        :.         .         .         ;'         6 

3.  A  vertical  dyke        .%        .         .    •     .         .         .         ;         .    •     .       13 

4.  A  laccolith        .         .    .     .         .         .         ,       ".'        ...•'•         .       14 

5.  A  volcanic  neck        .         .         .         .         .         ...         .15 

6.  Open  joints  caused  by  bending  of  strata        .         .         .         .       19 

7.  Anticlines  and  synclines  .         .         .         .         .         .         .         .22 

8.  A  recumbent  fold .22 

9.  An  anticlinorium      .         .         .         .         .         .         ...       22 

10.  Simple  unconformity .23 

11.  Diagram  of  a  fault  ......         .         .       24 

12.  Repetition  of  outcrop  by  faulting   .         .         •   '      •;        •         •       25 

13.  Normal  and  reversed  faults     .         .         .         .         .         ...       25 

14.  Fault  passing  into  a  thrust-plane 26 

15.  Suppression  of  outcrop  by  faulting          .         .         .         .         .       26 

16.  Unconformity  with  overstep 28 

17.  Unconformity  with  overlap 28 

18.  Diagram  of  dip-streams  .         .         ....         .         .         •         .64 

19.  Development  of  strike-streams 65 

20.  Highly  developed  river  system        ......       65 

21.  Profile  of  a  young  river  ........       66 

22.  Profile  of  an  old  river 66 

23.  Development  of  meanders 67 

24.  Ox-bow  lake 67 

25.  Cross  section  of  an  old  river  valley 68 

26.  Forms  of  valleys  due  to  water  and  ice-erosion       .         .         .       74 

27.  Roche  moutonne'e .       75 

28.  Forms  of  cliffs .         .       78 

29.  Cliff  with  wave-platform 78 

30.  The  Cambridge  Greensand .     102 

31.  Surface  quartzite      .         .         .         .         .         .         .         •-..'•     117 

32.  Formation  of  a  scree        ........     123 

33.  River  terraces  .         .         .       ^ .126 

34.  Formation  of  springs  in  horizontal  strata       .         .      .  .         .169 

35.  Formation  of  springs  in  inclined  strata  .         .         .         .         .170 


ILLUSTRATIONS  ix 

FIG.  PAGE 

36.  Spring  caused  by  a  dyke          .         .         .  .  .       -^  .     170 

37.  Spring  caused  by  a  fault         .        /.         .  .  .         .  .     171 

38.  Conditions  favourable  for  a  well    '.         .  .  '..        .  .     172 

39.  Well  subject  to  pollution         .         .  .  .         .  .172 

40.  Artesian  well    .         .                  .         .         .  .  .    .         .'  .     173 

41.  Method  for  determining  outcrop  of  strata  .  .         .  .     187 

42.  Vertical  faults .  ~;    •     .  .189 

43.  Diagram  of  dip  and  thickness  of  strata  .  ...  •     190 

44.  Teeth  of  Equus  and  Hipparion  (from  Lydekker's  The  Horse 

and  its  Relatives)  .         .         ..        ,:       .         .         .         .         .     305 

45.  The  ancestors  of  the  horse  (from  Lydekker's  The  Horse  and 

its  Relatives)       -    ...         .         .         .         .     •    .         .         .307 

46.  Feet  of   ancestors  of   the  horse  (from  Lydekker's  The  Horse 

and  its  Relatives)        ' ;  .  >       .     309 

47.  Skull  of  Urus  (from  Lydekker's  The  Ox  and  its  Kindred)      .     314 

48.  Skull  of  Mouflon  (from  Ewart,  in  Transactions  of  the  Highland 

and  Agricultural  Society)       .        .         .         .         .         .         .321 

49.  Skull  and  horns  of  Ammon  (from  Ewart,  in  Transactions  of 

the  Highland  and  Agricultural  Society)          .         .         .         .     322 

50.  Skull  of  bronze-age  sheep  (from  a  photograph  by  Ewart)      .     323 

51.  Skull   of    Dorset   ram    (from   Ewart,   in    Transactions   of   the 

Highland  and  Agricultural  Society)        .         .         .   ;      .         .     324 


CHAPTER   I 

INTRODUCTION.    MINERALS  AND   ROCKS 

« 

Definition  of  a  rock.  The  principal  obj  ect  of  the  agricultural 
geologist  is  the  study  of  the  soil,  but  since  all  soils  are  formed 
either  directly  or  indirectly  from  rocks,  it  is  essential  for  a 
proper  understanding  of  the  subject  to  gain  a  preliminary 
acquaintance  with  the  composition  and  properties  of  the  rocks 
themselves,  which  are  the  ultimate  source  of  the  materials  of 
the  soil.  This  material  has  as  a  rule  undergone  many  vicissi- 
tudes before  attaining  the  condition  in  which  it  is  found  at  the 
present  time.  All  geological  processes  work  in  cycles  of  change, 
which  commonly  have  no  definite  beginning  and  no  definite 
end;  hence  it  becomes  necessary  to  adopt  some  more  or  less 
arbitrary  starting-point  in  any  discussion  of  the  processes  which 
are  continuously  taking  place.  Rocks  are  continually  being 
formed  and  again  destroyed;  they  are  undergoing  ceaseless 
change,  but  these  changes  are  generally  very  slow,  and  it  is 
often  possible  to  find  a  rock  in  what  is  technically  known  as  a 
fresh  condition,  this  term  being  used  to  signify  the  fact  that  it 
has  undergone  little  or  no  visible  alteration  since  its  consolida- 
tion into  its  present  form.  It  is  convenient  therefore  to  begin 
with  a  study  of  fresh  rocks,  from  the  mineralogical  and  chemical 
standpoint,  and  to  leave  the  investigation  of  subsequent 
changes  to  a  later  stage. 

To  give  an  accurate  scientific  definition  of  a  rock  is  some- 
what more  difficult  than  would  appear  at  first  sight.  The 
term,  as  popularly  applied,  generally  connotes  the  ideas  of 
solidity  and  hardness,  but  this  criterion  soon  breaks  down 
when  applied  in  practice,  since  rocks  vary  greatly  in  hardness 

n.  A.  G.  1 


\,      INTRODUCTION  [CH. 

every1  transition  may  be  found  from  the  hardest  granites 
and  quartzites  to  forms  which  are  actually  quite  unconsolidated 
and  incoherent.  Even  among  the  older  strata  composing  the 
earth's  crust  and  at  considerable  depths  below  the  surface, 
there  may  be  found  beds  of  loose  sand  and  other  materials, 
differing  little  from  the  modern  deposits  of  the  sea-shore  or  the 
superficial  accumulations  of  the  land-surfaces.  To  the  geologist 
all  these  ancient  deposits  are  rocks,  whatever  their  state  of 
aggregation,  and  in  passing  upwards  from  older  to  newer  deposits 
it  is  impossible  to  fix  on  any  definite  horizon  at  which  rocks 
cease  and  surface  accumulations  begin;  in  fact  under  certain 
conditions  the  most  recent  formations  may  be  as  solid  and 
hard  as  anything  which  has  been  formed  in  the  past.  From 
this  point  of  view  rocks  may  be  said  to  include  all  the 
solid  materials  forming  the  crust  of  the  earth,  so  far  as  it 
is  accessible  to  observation. 

Perhaps  however  the  most  satisfactory  definition  of  a  rock 

is  an  aggregate  of  mineral  particles.     This  definition  makes  no 

assumption  as  to  the  composition  or  state  of  aggregation  of  the 

particles  and  hence  may  be  taken  to  include  the  accumulations 

of  all  ages  and  of  all  degrees  of  solidification  and  alteration, 

without  regard  to  their  manner  of  formation.     As  will  appear 

more  fully  in  a  later  section,  in  some  cases  rocks  are  formed  by 

consolidation  from  a  state  of  fusion,  while  in  other  cases  they 

originate  at  or  near  the  surface,  under  normal  conditions  of 

temperature  and  pressure,  through  the  action  of  the  ordinary 

geological  agents,  but  the  above  definition  holds  in  either  case. 

As  will  be  explained  more  fully  in  a  later  section,  rocks  may 

be  divided  into  two  well-marked  classes,  differing  fundamentally 

in  their  mode  of  origin.     These  are:   (a)   the  igneous  rocks, 

formed  by  consolidation  from  a  state  of  fusion — an  example 

of  this  class  is  afforded  by  the  lava  emitted  during  a  volcanic 

eruption ;  (6)  the  sedimentary  rocks,  formed  on  the  earth's  surface 

or  in  water  by  the  operations  of  ordinary  geological  agents. 

These  are  built  up  either  directly  or  indirectly  from  the  materials 

of  pre-existing  rocks  and  the  ultimate  source  of  the  materials 

is  to  be  sought  in  the  igneous  group.     Such  are  the  sandstones, 

clays  and  other  common  types  which  form  the  land-surface 


i]  MINERALS  AND   ROCKS  3 

of  a  great  part  of  the  world.  Under  the  heading  of  metamorphic 
rocks  are  included  all  those  varieties  which  have  undergone 
since  their  deposition  such  changes  that  they  have  assumed 
new  characters ;  in  some  cases  these  changes  are  so  far-reaching 
that  it  is  difficult  or  impossible  to  decide  whether  the  rocks 
were  originally  igneous  or  sedimentary. 

Chemical  composition  of  rocks.  The  rocks  composing  the 
earth's  crust  must  obviously  contain  all  the  known  elements, 
some  eighty  in  number,  with  the  possible  exception  of  a  few 
existing  only  in  the  atmosphere,  but  the  number  of  elements 
which  occur  in  large  proportion  is  very  small.  According  to 
the  most  recent  computations  only  eight  elements  occur  in 
amounts  exceeding  1  per  cent,  of  the  whole,  namely,  oxygen, 
silicon,  aluminium,  iron,  calcium,  magnesium,  sodium  and 
potassium.  These,  together  with  carbon,  sulphur,  phosphorus, 
titanium,  hydrogen  and  chlorine  form  all  the  common  rock- 
forming  minerals. 

The  results  of  analysis  of  rocks  and  minerals  are  generally 
calculated  not  as  elements,  but  as  oxides,  and  when  stated  in  this 
way  the  composition  of  the  earth's  crust,  or  lithosphere,  is 
approximately  as  follows1 : 

Per  cent. 

Silica,  SiO2           59-85 

Alumina,  A12O3 14-87 

Ferric  oxide,  Fe2O3         2-63 

Ferrous  oxide,  FeO         3-35 

Magnesia,  MgO 3-77 

Lime,  CaO            4-81 

Soda,  Na2O           3-29 

Potash,  K2O         3-02 

Water,  H2O          2-05 

Titanium  dioxide,  TiO2 -73 

Carbon  dioxide,  CO2       ...         ...  -70 

Phosphorus  pentoxide,  P2O5     ...  -25 

Sulphur,  S             -10 

Chlorine,  Cl          -06 

99-48 

1  Clarke,  "The  Data  of  Geochemistry,"  2nd  edition,  Bulletin  491,  United 
States  Geological  Survey,  1911,  p.  32. 

1—2 


4  INTRODUCTION  [CH. 

From  these  figures  it  will  be  seen  that  the  percentage  of 
certain  substances,  such  as  carbon,  phosphorus,  sulphur  and 
chlorine,  is  very  small,  but  they  are  all  elements  of  much 
scientific  and  practical  importance  and  are  therefore  included. 
The  proportion  of  the  valuable  metals,  gold,  silver,  copper,  tin, 
lead,  etc.,  is  insignificant  as  compared  with  the  substances 
above  enumerated.  The  element  titanium,  though  very  widely 
distributed,  is  of  no  practical  importance. 

Rock-forming  minerals.  It  has  already  been  stated  that 
the  constituents  of  rocks  are  minerals,  and  it  now  becomes 
necessary  to  understand  the  meaning  of  this  term.-  A  mineral 
may  be  defined  as  a  naturally  occurring  inorganic  substance, 
possessing  definite  physical  properties  and  in  most  cases  a 
definite  crystal-form,  though  certain  true  minerals  occur  in  an 
amorphous  or  non-crystalline  condition.  In  many  instances 
the  composition  of  minerals  can  be  expressed  by  simple  chemical 
formulae,  since  they  are  the  crystalline  forms  of  pure  chemical 
compounds  (e.g.  quartz,  Si02;  calcite,  CaC03;  magnetite, 
Fe304),  or  even  as  elements  (e.g.  diamond,  C;  sulphur,  S,  etc.). 
But  many  of  the  most  important  rock-forming  minerals  are 
not  pure  chemical  compounds;  they  are  to  be  regarded  rather 
as  mixtures  of  various  compounds  possessing  the  property  of 
isomorphism ;  in  other  words  they  are  mixed  crystals.  Under 
this  heading  come  most  of  the  silicates,  a  group  of  minerals  of 
the  highest  importance  as  constituents  of  rocks  and  soils. 

The  total  number  of  mineral  species  which  have  been 
recognized  by  systematic  mineralogists  is  enormous  and  ever 
increasing,  but  fortunately  only  a  very  small  proportion  of 
these  are  of  practical  importance  to  the  geologist.  It  is  possible 
to  draw  up  a  list  of  less  than  twenty  minerals,  which  together 
constitute  99  per  cent,  of  the  whole  visible  crust  of  the  earth. 
For  most  purposes  the  rest  may  be  disregarded. 

The  following  list  comprises  the  varieties  which  are  of  most 
common  occurrence  as  rock-builders;  some  of  them  can  be 
formed  only  by  crystallization  from  a  fused  state,  while  others 
can  only  originate  at  the  ordinary  temperature  and  pressure, 
that  is,  as  sedimentary  deposits.  A  few  are  common  to  both 
groups. 


I] 


MINERALS  AND   ROCKS 


Quartz 

Felspar 

Mica 

Hornblende 

Augite 


Olivine 
Magnetite 
Iron  pyrites 
Calcite 
Dolomite 


Rock-salt 
Gypsum 
Apatite 
Garnet 


The  student  is  recommended  to  make  himself  thoroughly 
familiar  with  the  chemical  composition  and  physical  characters 
of  these  minerals;  reading  should  be  supplemented  by  actual 
examination  of  well-selected  specimens,  until  the  different 
varieties  can  be  readily  recognized  by  inspection,  or  by  an 
application  of  the  simple  tests  described  in  any  text-book  of 
mineralogy1. 

Quartz  is  the  commonest  of  the  crystalline  forms  of  silicon 
dioxide  or  silica,  Si02.  It  crystallizes  in  the  hexagonal  system, 
most  usually  in  the  form  of  a  six- 
sided  prism  surmounted  by  a  six-sided 
pyramid.  It  is  also  frequently  found 
as  irregular  masses,  or  as  aggregates 
of  small  crystals  and  crystalline  grains. 
It  possesses  no  regular  cleavage,  but 
breaks  with  an  irregular  and  often 
curved  fracture.  The  colour  is  very 
variable ;  some  specimens  are  perfectly 
colourless,  clear  and  transparent,  while 
other  varieties  are  opaque  and  milky 
in  appearance,  or  show  various  shades 
of  brown,  yellow,  pink,  or  purple ;  the 
variations  in  colour  are  due  to  the 
presence  of  minute  quantities  of  some 
impurity.  Quartz  is  very  hard,  scratch- 
ing glass  easily,  and  its  specific  gravity 
is  about  2-65.  It  is  totally  unaffected 
by  any  acids,  except  hydrofluoric  acid, 
and  is  one  of  the  most  stable  and  resistant  of  all  minerals. 

Note.     Silica  occurs  widely  in  nature  in  other  forms,  differ- 
ing from  quartz  in  their  physical  properties.     They  are  either 
obscurely  crystalline   or   amorphous,  forming  such  substances 
1  Hatch,  Mineralogy,  4th  edition,  London,  1912. 


Fig.  1.  A  simple  crystal  of 
quartz,  consisting  of  a  hexa- 
gonal prism,  terminated 
by  hexagonal  pyramids. 


INTRODUCTION 


[CH. 


as  opal,  chalcedony,  jasper  and  flint.  Most  of  these  possess 
a  lower  specific  gravity  than  quartz  (opal,  2-1)  and  are  more 
easily  attacked  by  acids  and  especially  by  alkalies. 

Felspar.     Under  this  heading  are  comprised  a  large  number 
of  minerals  to  which  systematic  mineralogists  apply  different 

names.  Their  chemical  composi- 
tion is  rather  complex,  but  all 
contain  silica,  alumina,  and  either 
potash,  soda  or  lime,  or  some 
mixture  of  the  three  latter  bases. 
For  convenience  they  may  be 
divided  into  the  potash-felspar  or 
orthodase  group,  in  which  potash 
is  dominant,  and  the  soda-lime  or 
plagioclase  group,  containing  soda 
or  lime,  or  a  mixture  of  the  two. 
These  two  groups  differ  slightly 
in  physical  properties,  though  their 
general  appearance  is  very  similar. 
The  chemical  composition  of 


Fig.  2.     A  typical  simple  crystal 
of  orthoclase  felspar. 


orthoclase  may  be  expressed  by  either  of  the  formulae,  KAlSi308, 
or  K20.  Al203.6Si02.  The  latter  form  is  to  be  preferred.  This 
corresponds  to 


Silica 

Alumina... 
Potash    . 


Per  cent. 
64-7 
184 
16-9 


100-0 

Hence  the  percentage  of  potash  is  seen  to  be  high,  and  in  fact 
orthoclase  is  one  of  the  most  important  sources  of  potassium 
in  soils. 

The  felspars  of  the  plagioclase  group  may  be  regarded  as 
mixtures  in  any  proportion  of  the  two  ideal  compounds: 
albite,  Na20  .  A1203  .  6Si02,  and  anorthite,  CaO  .  A1203  .  2Si02. 

Albite  Anorthite 

Silica            ...         68-7  43-2 

Alumina       ...         19-5  36-7 
Soda             ...         11-8 

Lime             ...  20-1 

100-0  100-0 


i]  MINERALS  AND   ROCKS  7 

Special  names  have  been  applied  to  many  intermediate  varieties, 
but  for  our  purpose  it  is  unnecessary  to  enumerate  these. 

Orthoclase  crystallizes  in  the  oblique,  and  plagioclase  in  the 
anorthic  system.  The  general  forms  of  the  crystals  are  however 
very  similar.  All  felspars  possess  two  very  perfect  cleavages, 
which  are  at  right  angles  in  orthoclase  and  nearly  so  in  plagio- 
clase. The  cleavage  faces  generally  show  a  conspicuous  pearly 
lustre.  The  Colour  is  variable;  generally  white  or  grey,  or  a 
pale  shade  of  pink  or  red;  green  varieties  are  less  common. 
Occasionally  a  peculiar  bluish  iridescence  is  noticeable.  The 
mineral  is  hard,  though  softer  than  quartz,  and  the  specific 
gravity  varies  from  2-6  to  2-7,  the  varieties  rich  in  lime  being 
the  heavier. 

The  felspars  show  very  conspicuously  the  peculiar  structures 
known  as  twinning.  For  an  explanation  of  this  term  a  text-book 
of  mineralogy  must  be  consulted.  It  will  suffice  here  to  say 
that  this  phenomenon  affords  a  ready  means  of  distinction 
between  orthoclase  and  plagioclase,  since  most  of  the  faces  of 
the  latter  mineral  are  marked  by  fine  parallel  striations  due  to 
twinning  of  a  kind  which  cannot  occur  in  orthoclase.  The 
presence  of  these  striations  affords  an  infallible  diagnosis  for 
plagioclase. 

Mica.  The  micas  constitute  a  group  of  minerals  of  rather 
complex  chemical  composition,  possessing  well-marked  physical 
characters  which  render  their  identification  easy.  They  are 
silicates  containing  alumina  and  one  or  more  of  the  following 
bases :  potash,  soda,  iron  or  magnesia.  Those  varieties  rich 
in  iron  are  deeply  coloured,  usually  brown,  and  may  be 
designated  biotite,  while  the  varieties  without  iron  are  pale  or 
colourless  and  may  be  placed  under  the  heading  muscovite. 
All  micas  contain  more  or  less  hydrogen,  which  probably  exists 
in  combination  as  the  radical  OH.  The  composition  of  an 
ideal  muscovite  may  be  provisionally  represented  by  the 
formula  H20  .  K20  .  2A1203  .  4Si02.  The  constitution  of  biotite 
is  even  more  complex  than  this. 

All  the  micas  possess  one  very  perfect  cleavage  and  show 
a  strong  tendency  to  form  thin  flakes  with  a  brilliant  metallic 
lustre.  Muscovite  is  a  remarkablv  stable  mineral  and  is  not 


8  INTRODUCTION  [OH. 

affected  by  agents  of  decomposition,  being  therefore  abundant 
in  sediments  as  well  as  in  igneous  rocks,  whereas  biotite  is 
easily  decomposed  by  weathering  and  hence  is  characteristic  of 
fresh  rocks,  both  igneous  and  metamorphic. 

Hornblende  is  a  silicate  of  magnesia,  iron  and  lime  in  varying 
proportions;  some  varieties  also  contain  soda.  It  usually 
forms  prismatic  or  needle-shaped  crystals  with  an  approximately 
hexagonal  cross  section.  There  are  two  good  cleavages,  which 
cross  at  angles  of  124°  and  56°.  Most  varieties  of  hornblende 
are  black  in  colour,  occasionally  green,  with  a  semi-metallic 
or  somewhat  resinous  lustre.  The  mineral  is  hard  and  heavy 
(specific  gravity  about  3-1).  It  is  stable  and  not  attacked  by 
acids.  Hornblende  is  common  in  igneous  and  metamorphic 
rocks,  but  only  occurs  in  sediments  as  derived  grains. 

Augite  is  very  similar  to  hornblende  in  most  of  its  properties, 
the  chemical  composition  being  almost  identical.  The  crystals 
differ  somewhat  in  form,  usually  showing  eight  prism  faces 
instead  of  six,  and  the  two  cleavages  are  nearly  at  right  angles. 
The  colour  and  lustre  are  similar,  but  augite  is  heavier  than 
hornblende,  having  a  specific  gravity  of  about  3-3.  It  is 
common  in  igneous  rocks  and  is  somewhat  more  liable  to 
decomposition  than  hornblende,  being  therefore  less  abundant 
in  sediments. 

Olivine  is  a  silicate  of  iron  and  magnesia ;  it  may  be  regarded 
as  a  mixture  of  the  two  compounds  Mg2Si04  and  Fe2Si04. 
It  occurs  most  commonly  as  crystalline  aggregates  and  rounded 
grains,  of  a  yellowish  green  colour.  "It  shows  little  or  no 
cleavage  and  a  glassy  lustre.  Olivine  is  unstable  and  is  only 
found  in  the  igneous  rocks. 

Magnetite  consists  of  the  magnetic  oxide  of  iron,  Fe304. 
It  forms  well-defined  crystals  which  are  generally  octahedral  in 
form,  or  it  may  occur  massive.  It  is  black  in  colour,  with  a 
brilliant  metallic  lustre,  and  the  density  is  high  (about  5*0). 
Magnetite  is  specially  common  in  igneous  rocks,  and  occurs 
also  in  sediments  to  a  considerable  extent. 

Iron  pyrites,  disulphide  of  iron,  FeS2,  is  an  easily  recognizable 
mineral  with  metallic  lustre  and  the  colour  of  brass;  it  is 
very  hard  and  heavy,  and  most  commonly  occurs  in  the  form 


i]  MINERALS  AND   ROCKS  9 

of  cubes.  It  is  specially  characteristic  of  sedimentary  rocks  of 
a  muddy  nature,  which  have  been  subjected  to  a  certain  amount 
of  metamorphism. 

Calcite  is  the  more  important  of  the  two  crystalline  forms 
of  calcium  carbonate,  CaC03.  Its  crystalline  forms  are  very 
numerous  and  variable,  all  being  referable  to  the  rhombohedral 
subdivision  of  the  hexagonal  system.  It  possesses  three  very 
perfect  cleavages  arranged  in  the  form  of  a  rhombohedron. 
Cleavage  faces  show  a  strong  pearly  lustre.  The  colour  is 
variable,  but  generally  pale;  some  varieties  are  clear  and 
colourless  (Iceland  spar),  while  others  are  milky  white,  yellowish 
or  pink.  Calcite  is  soft,  being  easily  scratched  with  a  knife,  and 
it  effervesces  readily  with  dilute  hydrochloric  acid.  Specific 
gravity  2-72. 

Dolomite  is  a  double  carbonate  of  calcium  and  magnesium, 
CaMg(C03)2.  Its  crystalline  forms  are  similar  to  those  of  calcite, 
but  simpler,  and  the  crystals  often  show  curved  faces.  The 
cleavages  also  are  similar  to  those  of  calcite.  The  colour  is 
generally  creamy  white,  yellowish  or  pale  brown,  colourless 
varieties  not  being  very  common.  Dolomite  has  the  same 
hardness  as  calcite,  but  does  not  effervesce  readily  unless  the 
acid  is  warmed.  Specific  gravity  2-85. 

Rock-salt,  sodium  chloride,  NaCl,  is  sometimes  found  as 
cubic  crystals,  but  is  more  common  in  the  massive  form,  in 
layers  and  beds  in  stratified  rocks.  It  has  no  cleavage,  and  is 
very  soft,  being  easily  scratched  by  the  finger-nail.  It  is 
readily  soluble  in  water,  and  can  be  identified  by  the  taste. 
When  pure  it  is  colourless,  but  is  generally  stained  red  or  brown 
by  an  admixture  of  fine  clay. 

Gypsum  is  crystallized  calcium  sulphate,  CaS04 .  2H20. 
It  occurs  either  in  oblique  crystals  disseminated  through  beds 
of  clay,  or  in  a  fibrous  massive  form ;  the  latter  is  often  called 
satin  spar.  Gypsum  possesses  one  very  perfect  cleavage  and 
two  less  well-developed.  It  is  even  softer  than  rock-salt,  but 
is  only  slightly  soluble  in  water.  It  does  not  effervesce  with 
acids. 

Apatite  is  the  crystalline  form  of  calcium  phosphate.  It 
occurs  very  commonly  in  the  igneous  rocks  in  small  hexagonal 


10  INTRODUCTION  [CH. 

crystals.  It  is  generally  colourless,  sometimes  green  or  brown 
when  in  large  crystals,  fairly  hard  and  heavy  (specific  gravity 
3-2).  Apatite  is  of  great  practical  importance  as  the  ultimate 
source  of  the  phosphoric  acid  of  the  soil. 

Garnet  is  a  silicate,  usually  containing  several  metallic 
radicals,  of  which  alumina  is  often  present  in  large  proportion. 
It  crystallizes  in  complex  forms  belonging  to  the  cubic  system, 
and  varies  in  colour  from  pink  to  brown  and  black.  It  is  hard 
and  possesses  a  brilliant  lustre,  with  high  refractive  index ; 
hence  it  is  sometimes  employed  as  a  gem-stone.  Garnet  is 
most  characteristic  of  metamorphic  rocks,  though  it  also  occurs 
in  those  of  purely  igneous  origin.  It  is  also  a  common  derived 
constituent  of  sands. 

Besides  the  minerals  enumerated  above,  many  others  are 
common  as  constituents  of  igneous  and  metamorphic  rocks, 
of  sediments  and  of  the  soils  derived  from  them.  Most 
of  them  are  of  little  or  no  practical  importance,  and  their  study 
belongs  rather  to  the  province  of  mineralogy  than  of  geology, 
and  by  the  agriculturist  at  any  rate  they  may  safely  be 
neglected.  Some  of  the  more  important  will  be  mentioned 
incidentally  at  a  later  stage  when  dealing  with  the  constituents 
of  soils. 

Classification  of  rocks.  Although  the  exact  nature  of  the 
geological  processes  which  took  place  in  the  earliest  stages  of 
the  earth's  history  is  still  for  the  most  part  a  matter  of  specula- 
tion, there  can  be  no  doubt  that  the  earliest  rocks  were  formed 
at  a  high  temperature,  and  they  must  have  resembled,  at  any 
rate  in  their  mineralogical  composition,  those  which  we  know 
to  have  solidified  at  later  periods  from  a  state  of  fusion.  It  is 
a  safe  inference  to  conclude  that  the  material  of  all  the  sedi- 
mentary rocks  was  derived  either  directly  or  indirectly  from 
this  primitive  crust,  and  perhaps  to  some  extent  also  from  the 
primitive  atmosphere.  Disregarding  for  the  moment  this  latter 
possibility,  it  may  be  said  that  the  minerals  of  the  sediments 
are  derived  from  igneous  rocks,  and  therefore  the  study  of  rocks 
logically  begins  with  this  group,  although  at  the  present  time 
they  occupy  much  less  of  the  earth's  surface,  and  are  of  less 
importance  as  soil-formers. 


i]  MINERALS  AND   ROCKS  11 

The  igneous  rocks  comprise  all  those  masses  which  have 
solidified  from  a  state  of  fusion,  either  at  a  greater  or  less 
depth  within  the  earth's  crust,  or  actually  on  the  surface,  as  in 
the  case  of  lavas.  They  are  for  the  most  part  composed  of  an 
aggregate  of  crystals  and  grains  of  different  minerals  mixed  in 
different  proportions,  e.g.  quartz,  felspar,  mica,  hornblende, 
olivine,  etc.  The  deep-seated  rocks  are  always  completely 
crystalline,  but  in  some  cases  where  the  molten  material  found 
itself  at  or  near  the  surface,  cooling  was  so  rapid  that  crystals 
of  definite  minerals  could  not  form,  and  the  whole  solidified  as 
a  homogeneous  mass,  which  is  known  as  a  glass. 

The  second  great  group,  known  as  the  sedimentary,  aqueous 
or  stratified  rocks,  on  the  other  hand,  are  formed  at  the  ordinary 
surface  temperature  as  a  result  of  the  geological  processes  which 
may  be  seen  in  operation  around  us  at  any  time.  The  nature 
of  these  processes  will  be  fully  discussed  in  a  later  chapter ;  it 
must  suffice  here  to  say  that  as  a  result  of  the  slow  destruction 
of  the  land  by  water,  ice,  wind,  etc.,  masses  of  material  are 
loosened,  transported  by  various  agencies  and  piled  up  into 
masses,  which  may  eventually  become  consolidated  into  hard 
rocks,  or  may  remain  loose  and  incoherent  for  a  long  period. 
The  term  stratified  as  above  employed  connotes  the  fact  that 
these  masses  of  sediment  commonly  occur  in  layers,  or  strata, 
which  are  at  first  generally  horizontal,  but  may  eventually  be 
tilted  at  any  angle.  It  is  clear  that  the  sedimentary  rocks 
are  formed  from  the  materials  of  pre-existing  rocks,  with 
occasionally,  as  will  be  explained  later,  addition  of  matter  of 
organic  origin.  Even  this  latter  however  is  found  on  ultimate 
analysis  to  be  derived  from  rocks,  or  from  the  atmosphere.  The 
term  aqueous  is  frequently  used  as  synonymous  with  sedi- 
mentary, but  it  is  not  satisfactory,  since  there  are  important 
groups  of  sediments  in  whose  formation  water  plays  no  part. 

The  third  great  group  is  that  of  the  metamorphic  rocks; 
the  meaning  of  this  term  has  already  been  briefly  explained  and 
all  consideration  of  this  group  as  a  whole  may  be  deferred  for 
the  present. 

Rock-structures.  By  the  term  rock-structures  is  here 
understood  the  general  features  determining  the  forms  of 


12  INTRODUCTION  [OH. 

rock-masses  on  a  large  scale,  as  well  as  the  disposition  of  their 
divisional  planes  and  other  surfaces  of  discontinuity,  actual  or 
potential.  Rock-structures  are  the  characters  which  are  con- 
spicuous during  an  examination  of  rocks  in  the  field  and  they 
can  often  be  distinguished  at  considerable  distances,  as  for 
example  in  a  general  view  of  a  cliff  or  of  a  mountain.  The 
smaller  features  of  a  rock,  necessitating  observation  at  close 
range,  are  here  separated  under  the  heading  of  rock-textures. 
The  most  cursory  examination  of  any  section,  natural  or  arti- 
ficial, where  rock-masses  are  exposed  to  view,  will  generally 
disclose  more  or  less  of  a  parallel  arrangement ;  it  is  only  in  the 
case  of  some  igneous  rocks  that  this  is  entirely  absent,  and  even 
these  nearly  always  tend  to  break  up  into  blocks  bounded 
by  plane  or  curved  surfaces.  Completely  homogeneous  masses 
measuring  more  than  a  few  feet  are  rare. 

In  accordance  with  their  widely  differing  mode  of  origin  there 
is  a  good  deal  of  difference  in  the  structures  that  characterize 
the  igneous  and  sedimentary  rocks  respectively,  necessitating 
to  a  certain  extent  separate  treatment  for  each  group.  The 
metamorphic  rocks  also  possess  structures  peculiar  to  them- 
selves, which  are  direct  results  of  the  metamorphic  processes. 

Forms  assumed  by  igneous  rock-masses.  The  forms 
assumed  by  masses  of  molten  lava  poured  out  by  volcanoes 
will  evidently  depend  on  two  factors,  namely,  the  degree  of 
liquidity  of  the  lava,  and  the  form  of  the  surface  over  which  it 
flows.  A  liquid  lava  will  obviously  spread  out  into  a  thinner 
sheet  and  flow  further  than  a  viscous  one,  under  the  same  con- 
ditions, but  the  dominant  factors  are  the  form  and  slope  of  the 
surface  of  the  ground  over  which  they  flow;  since  these  may 
vary  indefinitely  it  is  not  possible  to  lay  down  any  general 
rules.  But  with  the  molten  masses  which  are  injected  into 
fissures  and  cavities  in  the  earth's  crust  it  is  quite  otherwise. 
The  degree  of  liquidity  is  here  also  of  some  importance,  but  it 
is  subordinate  to  the  form  of  the  space  into  which  the  material 
is  driven,  or  which  it  has  to  make  for  itself  by  its  own  kinetic 
energy.  The  form  in  this  case  will  be  largely  determined  by 
the  disposition  of  the  planes  of  weakness,  i.e.  of  the  divisional 
planes,  in  the  surrounding  rocks. 


I] 


MINERALS   AND   ROCKS 


13 


The  simplest  case  of  all,  perhaps,  is  when  the  molten  material 
merely  fills  a  crack  in  the  ground,  vertical  or  otherwise.  It 
may  or  may  not  reach  the  surface,  according  to  circumstances. 
Such  a  mass  is  called  a  dyke.  If  the  crack  reaches  the  surface 
molten  material  may  spread  out  as  a  thin  sheet,  just  as  with  a 
lava-flow;  in  fact  one  well-known  type  of  volcanic  eruption, 
the  fissure-eruption,  is  exactly  of  this  kind;  lava  wells  up 
through  a  crack  and  spreads  out  on  the  surface;  the  dyke  in 
this  case  is  the  feeder  of  the  flow.  Dykes  are  not  necessarily 
vertical  at  first  and  may  subsequently  be  tilted  at  any  angle. 
The  name  is  usually  restricted  to  intrusions  which  clearly  cut 
across  the  bedding  or  foliation  planes  of  the  surrounding  rocks. 
When  the  molten  material  forces  its  way  as  a  sheet  of  varying 


Fig.  3.     A  vei'tical  dyke7passing  up  into  a  sill  which  has  penetrated 
along  the  bedding  planes^of  the  sedimentary  rocks. 

thickness  along  the  bedding  planes  of  a  sediment  it  is  called  a 
sill.  Similarly,  more  or  less  horizontal  masses  cutting  across 
folded  and  crumpled  strata  are  called  sheets.  The  extent  and 
thickness  of  sills  vary  indefinitely ;  some  are  known  to  extend 
over  hundreds  or  even  thousands  of  square  miles,  e.g.  the  Whin 
Sill  of  the  north  of  England  and  the  Palisade  Traps  of  New 
York.  In  some  parts  of  the  world,  where  sills  are  very  abundant, 
they  are  the  determining  features  in  the  topography  and  hence 
in  the  economic  value  of  the  land.  In  the  stratified  rocks 
forming  the  Great  Karroo  in  South  Africa,  sills  are  extremely 
abundant,  and  by  their  superior  hardness  they  give  rise  to  the 


14  INTRODUCTION  [CH. 

peculiar  "kopje"  type  of  scenery,  consisting  of  steep,  terraced 
and  often  flat-topped  hills,  with  deep  valleys  between.  The 
kopjes  are  rocky  and  barren,  but  the  valleys  are  often  com- 
paratively fertile  and  well- watered. 

Among  the  large  deep-seated  intrusions  the  principal  forms 
are  the  laccolith,  the  bysmalith  and  the  boss.  The  first  of  these 
is  in  general  form  like  a  tea-cake,  and  the  strata  lying  above  it 
are  arched  up  into  a  dome ;  at  the  edges  a  laccolith  often  tails 


Fig.  4.     A  laccolith  fed  by  a  dyke  below;   the  overlying  strata 
being  lifted  up  to  form  a  dome. 

off  into  sills  and  more  or  less  vertical  offshoots  may  form 
dykes.  A  bysmalith  may  be  regarded  as  a  laccolith  of  very 
great  thickness  as  compared  with  its  horizontal  extent,  and  the 
overlying  strata  are  of  necessity  more  or  less  fractured.  A  boss 
is  a  somewhat  irregular  mass  of  very  great  size,  which  seems  to 
extend  indefinitely  downwards,  such  as  the  Dartmoor  granite 
mass. 

In  some  parts  of  the  world,  e.g.  Canada,  India  and  South 
Africa,  are  found  masses  of  igneous  rock,  especially  of  granite, 
covering  hundreds  or  thousands  of  square  miles,  and  of  unknown 
depth.  Such  masses  can  hardly  have  been  intruded  in  the 
ordinary  sense  of  the  word;  they  have  more  probably  been 
formed  by  bodily  fusion  in  situ  of  pre-existing  rocks  while  under 
a  very  thick  cover  of  later  rocks,  during  a  period  of  depression 
into  a  hotter  region  of  the  earth. 

A  fairly  common  special  form  of  igneous  mass  is  the 
neck.  This  is  in  point  of  fact  nothing  but  the  material  filling 
the  lower  part  of  the  pipe  or  vent  of  a  volcano,  which  has 


i]  MINERALS  AND   ROCKS  15 

solidified,  and  has  been  subsequently  exposed  at  the  surface  by 
the  removal  of  the  overlying  rocks.  In  consequence  of  their 
superior  hardness,  necks  often  stand  up  as  elevations  above  the 
softer  rocks  of  the  surrounding  country;  such  are  many  well- 
known  hills  in  the  central  lowlands  of  Scotland,  e.g.  North 
Berwick  Law,  Largo  Law,  Arthur's  Seat,  etc. 


Fig.  5.     A  volcanic  neck  which  has  been  denuded  and  now 
stands  up  to  form  a  hill. 

Besides  the  more  or  less  definite  forms  mentioned  above, 
igneous  rocks  often  occur  in  shapeless  masses  of  every  possible 
size,  their  forms  depending  for  the  most  part  on  the  arrangement 
of  the  planes  of  weakness  in  the  surrounding  rocks.  An  extreme 
case  is  where  molten  material  has  been  injected  in  numerous 
thin  sheets  along  the  bedding  planes  of  sediments  or  the  foliation 
planes  of  metamorphic  rocks.  This  constitutes  the  so-called 
lit-par-lit  injection,  which  is  common  in  many  areas  of  ancient 
crystalline  rocks,  giving  rise  to  banded  gneisses. 

Divisional  planes  in  rocks.  When  any  exposure  of  rocks  is 
examined,  whether  natural  or  artificial,  perhaps  the  first  thing 
to  be  noticed  is  that  the  masses  are  not  continuous.  In  almost 
every  case  the  rocks  are  broken  up  by  planes  of  discontinuity 
into  natural  blocks  of  very  various  shapes  and  sizes.  These 
divisional  planes  are  of  several  different  kinds  and  originate  in 
various  ways.  The  existence  and  arrangement  of  divisional 
planes  in  rocks  is  of  immense  practical  importance,  since  on 
it  depends  the  facility  or  otherwise  of  obtaining  blocks  of  stone 
of  convenient  sizes,  for  various  purposes,  besides  a  great 
influence  on  such  matters  as  mining,  water-supply,  drainage, 
etc. 

Some  divisional  planes  are  original,  having  been  in  existence 


16  INTRODUCTION  [CH. 

since  the  first  formation  of  the  rock,  while  others  are  secondary 
or  superinduced  as  the  result  of  various  disturbing  influences. 
Again  some  are  peculiar  to  igneous  rocks,  others  to  sediments 
or  metamorphic  rocks;  hence  it  becomes  necessary  to  treat 
them  systematically  and  in  detail.  It  may  be  mentioned  here 
that  some  divisional  planes  actually  exist  as  discontinuities  in 
the  undisturbed  rocks,  the  open  joints  of  the  quarryman,  while 
others  are  rather  planes  of  potential  division,  along  which  the 
rock  can  be  split  easily,  though  in  its  undisturbed  condition  it 
is  continuous;  an  example  of  this  is  afforded  by  ordinary 
roofing-slates. 

The  most  important  divisional  planes  of  rocks  are: 
(1)  stratification,  lamination  or  bedding ;  (2)  joints ;  (3)  cleavage, 
schistosity  and  foliation ;  (4)  folds ;  (5)  faults.  Of  these  groups 
the  third  and  fourth  are  always  secondary ;  the  second  generally 
so,  while  the  first  group  are  always  original. 

Stratification  and  bedding.  Since  the  sedimentary  rocks  are 
formed  by  accumulations  of  material  for  the  most  part  laid 
down  in  water,  but  sometimes  on  the  surface  of  the  land,  there 
is  naturally  a  strong  tendency  for  this  material  to  form  hori- 
zontal or  nearly  horizontal  layers.  Since  the  nature  of  the 
sediment  also  varies  from  time  to  time  there  is  often  an  alter- 
nation of  layers  of  different  composition,  texture  and  colour. 
To  express  these  relations  the  terms  stratification,  bedding 
and  lamination  are  employed.  The  distinctions  between 
these  three  terms  are  not  very  precise,  but  generally  speaking 
the  word  stratification  is  used  to  express  the  succession  on 
a  large  scale  of  different  kinds  of  sediment,  when  exposed 
to  view  over  a  large  surface,  such  as  a  cliff  or  a  deep  quarry; 
bedding  is  applied  to  successive  discontinuous  layers  of  sediment 
of  the  same  kind,  while  the  term  lamination  is  employed  for 
the  minute  divisional  planes,  sometimes  to  the  number  of 
hundreds  in  the  space  of  an  inch,  which  are  peculiar  to  certain 
fine- textured  rocks  (see  p.  87).  However,  the  employment  of 
these  terms  by  different  writers  shows  much  variation,  and  it 
is  obvious  that  no  sharp  distinction  can  exist  between  them. 

As  above  stated  the  layers  or  beds  of  sedimentary  rocks  are 
usually  at  first  horizontal,  but  they  do  not  always  remain  so. 


i]  MINERALS   AND   ROCKS  17 

As  a  result  of  movements  in  the  crust  of  the  earth,  often  accom- 
panied by  dislocation,  the  strata  are  tilted  in  various  ways, 
making  different  angles  with  the  horizontal  plane ;  it  therefore 
becomes  necessary  to  have  some  precise  method  of  expressing 
this  relation.  Stratified  rocks  which  are  inclined  to  the  hori- 
zontal plane  are  said  to  dip.  The  dip  of  a  rock-stratum  is 
the  steepest  line  which  can  be  drawn  in  the  plane  of  its 
stratification,  and  the  angle  of  dip  is  the  angle  which  this  line 
makes  with  the  horizontal,  measured  in  degrees.  This  fixes 
the  amount  of  the  dip,  but  it  is  also  necessary  to  find  some 
expression  for  its  direction.  This  is  expressed  in  terms  of  points 
of  the  compass.  Thus  when  a  bed  is  inclined,  e.g.,  to  the  north- 
east at  an  angle  of  35°,  this  may  be  written  shortly  as  "dip 
N.E.  35°."  If  the  direction  does  not  exactly  coincide  with  the 
major  points  of  the  compass,  it  may  be  expressed  as  for 
example  "10°  east  of  north,"  or  more  shortly  "dip  N.  10°  E." 
On  geological  maps  the  direction  and  amount  of  dip  is  shown 
by  a  small  arrow,  with  a  figure  alongside,  indicating  the  angle 
of  inclination  with  the  horizontal. 

The  strike  of  a  bed  is  a  horizontal  line  drawn  at  right  angle's 
to  the  dip.  Its  direction  is  also  expressed  with  reference  to  the 
points  of  the  compass.  For  example  the  full  definition  of  the 
position  of  a  certain  bed  might  be  as  follows :  "Strike  east  and 
west,  dip  30°  to  the  north,"  or  more  shortly,  "strike  E.-W.  dip 
30°  N." 

It  is  clear  that  if  a  series  of  uniformly  inclined  strata  come 
to  the  surface  on  level  ground,  they  will  intersect  the  ground  in 
straight  lines,  which  are  parallel  to  the  strike;  the  portion  of 
the  rocks  then  forming  the  surface  is  called  the  outcrop  of  the 
strata.  If  however  the  surface  is  undulating  the  form  of  the 
outcrop  will  also  be  curved  and  the  relations  become  more 
complex.  This  subject  cannot  here  be  pursued  further,  but 
will  be  dealt  with  again  in  the  chapter  on  geological  maps. 

Joints.  It  may  generally  be  observed  in  any  natural  or 
artificial  exposure  that  the  rocks  are  divided  into  blocks  of 
various  sizes  and  shapes  by  actual  discontinuities,  which  are 
sometimes  open  fissures  of  varying  width.  These  are  called 
joints.  They  are  found  both  in  igneous  and  in  sedimentary 

R.  A.  G.  9, 


18  INTRODUCTION  [OH. 

rocks,  and  may  come  into  existence  at  almost  any  period  of  the 
rock's  history.  Joints  may  be  conveniently  classified  into  two 
groups,  according  to  their  manner  of  origin,  namely  joints  due 
to  contraction,  and  joints  due  to  disturbance. 

When  a  mass  of  molten  igneous  rock  is  cooling  and  con- 
solidating it  undergoes  a  considerable  diminution  of  volume ; 
since  the  mass  is  as  a  rule  unable  to  contract  as  a  whole,  it  must 
of  necessity  break  up  into  separate  portions  owing  to  the  strains 
which  are  set  up  in  it.  If  the  strains  are  uniform  or  nearly  so, 
as  is  often  the  case,  the  blocks  thus  produced  will  also  tend  to 
be  uniform  in  size  and  shape.  Perhaps  the  best  example  of 
this  is  afforded  by  the  well-known  columnar  jointing,  so  charac- 
teristic of  certain  lavas  and  intrusive  sills.  Familiar  instances 
are  the  Giant's  Causeway  in  Antrim  and  Fingal's  Cave  in  the 
island  of  Staffa,  where  beds  of  igneous  rock  are  found  to  be 
broken  up  into  aggregates  of  well-formed  columns,  for  the  most 
part  hexagonal  in  shape,  though  some  have  five  or  seven  sides ; 
cross- jointing  is  also  conspicuous,  so  that  the  rock  tends  to 
break  up  into  blocks  rather  like  cheeses  in  form.  A  similar 
kind  of  columnar  structure  is  often  seen  in  layers  of  fine- textured 
homogeneous  sediment,  clay  or  mud,  which  have  been  dried 
by  exposure  to  the  air  and  sun.  In  all  cases  the  columns  are 
arranged  with  their  long  axes  perpendicular  to  the  surface  of 
cooling  or  of  drying. 

In  the  case  of  the  larger  intrusive  masses,  such  as  bosses 
and  laccoliths,  the  jointing  is  less  regular,  and  very  often  the 
principal  joints  tend  to  be  arranged  parallel  to  the  outer  surface 
of  the  intrusion ;  hence  if  this  is  curved,  so  also  will  the  joints 
be  curved.  Tabular  jointing  is  also  very  characteristic  of  many 
granite  masses,  as  in  Devon  and  Cornwall. 

Joints  in  stratified  rocks  are  due  to  two  principal  causes; 
contraction  on  drying,  and  disturbance  during  earth-movement. 
They  very  commonly  tend  to  arrange  themselves  in  three  sets 
at  right  angles  to  one  another,  breaking  up  the  strata  into  more 
or  less  cubical  blocks,  which  are  often  of  large  size.  In  undis- 
turbed strata  two  sets  of  these  principal  joints  or  major  joints 
are  vertical;  the  third  set  parallel  to  the  bedding  planes. 
Besides  these  there  are  nearly  always  minor  joints  of  Jess  regular 


MINERALS  AND   ROCKS 


19 


disposition.  The  presence  of  many  minor  joints  is  a  drawback 
in  building  stones,  as  they  spoil  the  shape  of  the  blocks.  When 
stratified  rocks  are  inclined,  the  major  joints  no  longer  remain 
vertical  and  horizontal,  but,  as  a  little  consideration  will  show, 
one  set  is  parallel  to  the  dip  of  the  rocks,  another  to  the  strike ; 
hence  they  are  called  dip-joints  and  strike-joints  respectively1. 

When  strata  are  tilted  from  the  vertical,  and  especially 
when  the  bedding  planes  are  bent  or  folded,  well-marked 
systems  of  joints  are  often  produced.  Rocks  are  not  ordinarily 
plastic,  but  rather  brittle,  and  they  fracture  readily  owing  to 
disturbance;  hence  joints  which  already  exist  are  widened, 
and  new  ones  are  produced.  In  extreme  cases  the  rocks  are 
much  shattered  and  broken  up  into  numerous,  often  shapeless 
blocks  of  all  sizes. 

Joints  which  originate  in  the  ways  above  mentioned  are 
often  widened  subsequently  by  weathering  and  especially  by 
solution,  eventually  forming  fissures  and  cavities  in  the  rocks. 
These  are  especially  prevalent  in  limestones,  which  are  more 
soluble  than  most  other  common  rocks. 


Fig.  6.     Development  of  open  joints  as  a  result  of  curvature  in 
stratified  rocks. 

Open  joints  are  particularly  liable  to  be  formed  in  well- 
stratified  rocks  which  have  been  bent  into  curved  forms,  as 
shown  in  Fig.  6,  since  the  strains  are  here  unequal  and  tension 
is  set  up ;  if  the  bending  is  sharp,  differential  movements  may 
take  place  along  the  bedding  planes,  and  as  explained  later, 
folds,  when  acute,  tend  to  pass  over  into  actual  discontinuities. 

1  Most  of  the  problems  connected  with  bedding,  dip,  strike  and  jointing 
in  stratified  rocks  can  be  well  illustrated  by  means  of  piles  of  books. 

2—2 


20  INTRODUCTION  [CH. 

Cleavage.  This  is  the  property  possessed  by  certain  rocks 
of  splitting  indefinitely  into  flakes  or  laminae,  which  are  all 
bounded  by  parallel  surfaces.  The  property  is  best  seen  in 
ordinary  roofing-slates,  which  owe  their  practical  value  to  its 
presence.  Cleavage  of  rocks  is  a  secondary  or  superinduced 
structure,  due  to  pressure,  and  it  therefore  belongs  properly  to 
the  domain  of  metamorphism.  Fine-textured  rocks,  such  as 
clay  or  mudstone,  consist  of  vast  numbers  of  minute  particles 
of  all  shapes,  but  many  of  these  possess  a  more  or  less  flattened 
or  elongated  form.  When  first  deposited  in  water  they  lie  in 
all  directions,  but  under  the  influence  of  pressure  they  are 
rearranged,  usually  with  their  long  axes  perpendicular  to  the 
direction  of  the  pressure.  It  is  evident  that  a  rock  possessing 
a  platy  structure  of  this  kind  will  split  more  readily  parallel 
to  the  flat  faces  of  the  fragments  than  in  a  direction  transverse 
to  this.  The  production  of  cleavage  is  also  assisted  by  the  fact 
that  under  pressure  a  certain  amount  of  mineralogical  change 
usually  takes  place,  leading  especially  to  the  formation  of  mica 
and  other  flaky  minerals,  which  split  readily.  Sometimes  the 
reconstruction  of  the  rocks  is  so  complete  that  all  trace  of 
original  structures  is  lost,  but  more  commonly  it  is  possible  to 
discern  the  original  bedding  planes,  which  are  often  made 
evident  by  differences  of  colour  or  of  texture.  Occasionally 
the  direction  of  cleavage  may  coincide  with  that  of  the  original 
bedding,  but  more  commonly  the  cleavage  makes  a  considerable 
angle  with  it,  since  the  pressures  which  produce  cleavage  are 
usually  horizontal  or  nearly  so  in  direction  and  due  to  com- 
pression or  crumpling  in  the  earth's  crust.  Care  must  be 
taken  to  distinguish  between  cleavage  in  rocks,  which  is  a 
secondary  structure  and  variable  in  direction,  and  cleavage  in 
minerals,  which  is  one  of  the  original  physical  properties  and 
is  constant  in  direction  in  all  specimens  of  the  same  mineral. 

Schistosity  and  foliation.  These  terms  are  applied  in  a 
somewhat  vague  manner  to  a  property  analogous  to  cleavage, 
developed  less  regularly  in  rocks  of  a  coarser  texture.  The 
structures  produced  are  generally  parallel  on  a  large  scale,  but 
less  regular  and  less  perfect  than  in  slates.  The  folia  also  are 
often  bent  and  contorted  in  various  ways,  so  that  flat  slabs 


ij  MINERALS   AND   ROCKS  21 

cannot  usually  be  obtained.  The  schistose  and  foliated  rocks 
have  commonly  undergone  a  good  deal  of  mineralogical  change 
and  are  often  highly  crystalline.  The  term  schist  is  generally 
applied  to  those  varieties  which  are  rather  fine  in  texture  and 
rich  in  mica  or  other  minerals  of  a  somewhat  silky  or  metallic 
appearance,  while  the  coarse-grained  foliated  varieties,  which 
are  rich  in  quartz  and  felspar,  are  called  gneisses,  but  no  hard 
and  fast  line  can  be  drawn  between  them.  The  term  foliation 
is  also  sometimes  applied  to  a  parallel  structure  in  igneous 
rocks,  which  is  not  due  to  pressure,  but  to  the  drawing  out 
during  flow  of  a  partly  consolidated  molten  mass  of  hetero- 
geneous composition  and  varying  colours.  This  may  also  be 
called  primary  gneissic  banding,  to  distinguish  it  from  the 
secondary  gneissic  banding  due  to  pressure.  The  schists  and 
gneisses  are  typical  examples  of  metamorphic  rocks. 

Folds.  The  crust  of  the  earth  has  been  in  times  past  and 
still  is  subjected  to  strain.  The  strains  mostly  take  the  form 
of  compression,  due  to  contraction  of  the  crust  as  a  whole. 
At  and  close  to  the  surface  this  compression  generally  leads  to 
fractures,  but  at  greater  depths  stratified  rocks  may  undergo 
crumpling,  the  originally  horizontal  bedding  planes  being 
doubled  up  into  arches  and  troughs  of  various  forms.  This 
process,  which  is  known  as  folding,  may  occur  on  any  scale  and 
of  any  degree  of  intensity.  Folds  on  the  largest  scale  form  the 
continents  and  ocean  basins ;  those  of  an  intermediate  character 
give  rise  to  mountain  ranges  and  other  geographical  features, 
while  folds  of  the  smaller  kind  grade  down  from  this  into 
sometimes  microscopic  contortions  of  bedding  planes,  such  as 
are  seen  in  certain  schists. 

When  the  strata  have  been  elevated  round  a  central  point  so 
that  they  dip  away  from  it  on  every  side,  this  is  called  a  dome, 
the  converse,  where  the  dip  is  towards  a  central  point,  being  a 
basin.  More  commonly  however  the  folding  takes  place  in 
elongated  areas,  the  central  line  of  such  an  area  being  called 
the  axis  of  the  fold.  Such  folds  possess  a  similarity  to  waves. 
A  simple  elongated  arch  is  called  an  anticline  and  the  corre- 
sponding trough  a  syncline.  Very  commonly,  a  folded  area  is 
made  up  of  a  series  of  parallel  arches  and  troughs,  which  may 


22 


INTRODUCTION 


[cu- 


be either  symmetrical  or  asymmetric.  Symmetrical  folds  are 
formed  when  the  pressure  is  equal  from  both  sides ;  if  it  is  much 
stronger  from  one  side  the  fold  becomes  asymmetric,  over- 
turned, or  recumbent,  according  to  the  degree  of  asymmetry. 


Fig.  7.     Stratified  rocks  folded  into  a  series  of  anticlines  and  synclines. 


Fig.  8.     A  recumbent  fold. 

All  of  these  varieties  are  most  easily  understood  from  the  figures. 
Another  term  commonly  employed  is  isoclinal  folding,  where 
both  limbs  of  an  overturned  fold  dip  in  the  same  direction. 
Folds  again  may  be  complex;  that  is  a  number  of  small 
folds  may  be  combined  into  the  general  form  of  one  large 
fold,  as  shown  in  the  figure  of  an  anticlinorium  (Fig.  9).  An 
exaggerated  form  of  an  anticlinorium  is  the  fan-structure  of 


Compound  anticline  or  anticlinorium. 


some  mountain  chains,  while  the  enormously  long  recumbent 
folds  of  the  Alps  may  also  be  mentioned. 

Highly-folded  rocks  commonly  form  mountain  regions,  at 
any  rate  at  first,  while  those  which  are  less  folded  constitute 
regions  of  lower  relief.  It  has  frequently  happened  however 
that  in  course  of  time  elevated  areas  have  been  worn  down  by 


I] 


MINERALS   AND   ROCKS 


23 


denudation  till  they  constitute  a  plain,  which  may  eventually 
be  overflowed  by  the  sea,  or  by  a  lake,  or  covered  up  by 
terrestrial  deposits.  In  such  a  case  the  newer  sediments  will 
rest  on  the  denuded  edges  of  the  folded  rocks  below,  producing 
a  strongly  marked  unconformity,  and  such  are  very  common 
in  the  older  strata.  It  is  quite  clear  that  in  the  case  of  an 
unconformity  the  rocks  below  may  be  and  often  are  much 
more  folded  than  those  above,  while  the  contrary  case  is 
impossible. 


-H-r1 


-w- 


Fig.  10.     Simple  unconformity;   horizontal  strata  resting  on  folded  strata. 

The  importance  of  the  occurrence  of  rock-folds  from  the 
practical  point  of  view  is  easily  demonstrated,  especially  in 
mining  geology,  but  it  is  also  of  much  significance  in  other 
ways.  Strongly  developed  folding  frequently  leads  to  the 
outcrop  of  the  same  bed  at  the  surface  many  times  within  a 
limited  area,  and  this  may  have  much  influence  on  the 
topography  of  the  ground,  the  character  of  the  soil,  the  water 
supply  and  the  natural  drainage. 

Faults.  As  already  explained  rocks  have  often  been  sub- 
jected to  strains  as  a  result  of  earth-movements.  Under  some 
conditions  these  strains  result  in  crumpling  and  folding  of  the 
crust,  but  in  other  circumstances,  the  strata  may  be  fractured 
instead  of  folded.  Again  it  sometimes  happens  that  fracture 
is  the  result  of  folding  carried  beyond  the  limits  of  plastic 
deformation,  folds  thus  passing  over  into  faults.  Faults  are  of 
many  kinds,  but  the  essential  feature  is  differential  movement 
between  blocks  of  the  crust,  along  a  plane  of  discontinuity. 
Faults  may  be  vertical  or  inclined  at  any  angle.  When  vertical, 
the  displacement  or  distance  between  the  broken  ends  of  the 


24  INTRODUCTION  [CH. 

same  bed  is  called  the  throw  of  the  fault.  When  the  fault  is 
inclined,  the  displacement  along  the  fault-plane  may  be  regarded 
as  made  up  of  two  components,  the  vertical  one  being  called 
the  throw  and  the  horizontal  one  the  heave  of  the  fault.  The 
inclination  of  the  fault  is  most  conveniently  measured  by  the 
angle  that  it  makes  with  the  horizontal,  or  dip,  as  in  the  case 
of  stratified  rocks,  although  it  has  till  lately  been  the  custom 
to  measure  the  angle  from  the  vertical;  it  is  then  called  the 
hade  of  the  fault.  All  these  relations  are  shown  in  Fig.  11. 


A' 


B' 


Fig.  11.  VZ  =  horizontal  plane,  X  7  =  fault  -plane.  AA',  BB'  =  a  hori- 
zontal stratum  of  rock.  AB  —  displacement  of  fault,  AC  =  throw,  EC  —  heave. 
VXY  =  ABC  =  dip  of  fault,  B A C  —  hade  of  fault.  BB'  is  on  the  downthrow 
side.  AA'  on  the  upthrow  side. 

It  is  necessary  also  to  have  some  means  of  expressing  the  relative 
movement  of  the  two  crust-blocks;  the  one  which  is  moved 
upwards  or  away  from  the  earth's  centre,  relatively  to  the  other 
is  called  the  upthrow  side  of  the  fault,  and  the  other  the  down- 
throw side;  as  a  rule  it  is  not  possible  to  determine  which 
block  actually  moved ;  it  is  only  the  relative  displacement  that 
can  be  measured. 

When  the  fault  is  vertical  and  the  strata  horizontal  the 
relations  are  very  simple,  the  only  quantity  required  to  be 
determined  being  the  throw  or  vertical  displacement.  With 
inclined  strata  also  there  is  little  difficulty ;  the  effect  of  faulting 
may  be  a  repetition  of  the  outcrops  of  certain  strata,  as  shown 
in  Fig.  12.  When  both  strata  and  fault  are  inclined,  and  in 
some  cases  even  with  vertical  faults,  the  outcrops  of  some  strata 
may  be  suppressed  altogether,  as  can  be  seen  from  a  study  of 


I] 


MINERALS  AND   ROCKS 


25 


the  figures.     The  possible  cases  not  here  figured  can  easily  be 
worked  out  by  the  student. 


Fig.  12.     The  outcrop  of  inclined  strata  repeated  by  a  fault. 

When  an  inclined  fault  dips  towards  the  downthrow  side 
(the  commonest  case)  the  fault  is  said  to  be  normal.  When  on 
the  other  hand  one  block  has  been  as  it  were  pushed  up  the 
fault-plane  this  is  called  a  reversed  fault.  In  this  case  it  is 
always  possible  by  sinking  a  vertical  shaft  to  pierce  some  of  the 
strata  twice  and  this  may  be  of  importance  in  mining.  On  the 
other  hand  in  the  case  of  a  normal  fault  a  shaft  may  miss  a 
particular  stratum  altogether  (see  Fig.  13). 


Fig.  13.     A—  normal  fault,  B  =  reversed  fault.     The  dotted  line  in  each 
case  indicates  a  vertical  shaft  or  boring  cutting  the  fault-plane. 

As  already  stated  a  fault  may  be  inclined  at  any  angle ;  in 
some  instances  of  reversed  faults  the  inclination  approaches  the 
horizontal.  Such  a  fault  is  commonly  called  a  thrust-plane. 
In  extreme  cases  the  displacement  may  be  measured  by  miles. 
A  thrust- plane  commonly  results  from  fracture  along  the  middle 
limb  of  a  recumbent  fold,  when  the  strain  exceeds  the  elastic 
limit  of  the  rocks. 


26 


INTRODUCTION 


CH. 


Since  faults  frequently  result  from  excessive  folding  their 
position  often  has  a  definite  relation  to  the  direction  of  the 
folds;  they  are  generally  either  parallel  or  perpendicular  to 
the  strike  of  the  rocks,  and  are  therefore  called  strike-faults 
and  dip-faults. 


Fig.  14.     Recumbent  fold  passing  into  thrust-plane. 

Each  of  these  naturally  produces  a  characteristic  effect  on 
the  outcrops;  strike-faults  cause  doubling  or  suppression  of 
outcrops  as  before  explained  (see  Figs.  12  and  15).  The  chief 
effect  of  dip-faults  is  to  cause  an  apparent  lateral  shifting 
of  the  outcrops.  The  subject  is  complicated  and  cannot  be 
pursued  here  owing  to  considerations  of  space. 


Fig.  15.  Suppression  of  outcrop  by  faulting.  The  dotted  lines  at  A'  show 
the  original  continuation,  before  denudation,  of  the  bed  A,  which  now  does 
not  come  to  the  surface. 

Hitherto  it  has  been  assumed  in  the  case  of  vertical  faults 
that  the  movement  was  strictly  vertical,  but  this  is  not  always 
so.  There  may  also  be  differential  movement  between  the 


i]  MINERALS   AND   ROCKS  27 

crust  blocks  in  a  horizontal  direction,  or  the  displacement  may 
even  be  exclusively  of  this  nature.  Here  also  there  will  evidently 
be  lateral  shifting  of  the  outcrops  in  inclined  strata. 

In  nature  fault-planes  are  not  as  a  rule  perfectly  clean-cut 
and  straight;  they  are  often  curved  or  undulating.  During 
the  movement  of  the  blocks  also  minor  projections  are  often 
broken  away,  and  a  certain  amount  of  fracturing  and  rolling  of 
material  may  take  place.  Hence  fault-fissures  are  often  filled 
with  fragmental  material,  known  as  fault-breccia.  Again  fault 
fissures  often  serve  as  channels  for  the  passage  of  solutions,  and 
these  may  deposit  material  from  solution  in  the  fissure.  Hence 
faults  are  frequently  filled  with  secondary  material,  sometimes 
including  many  minerals  of  economic  value.  Most  of  the 
metallic  ore-veins  of  Cornwall,  for  example,  are  found  to  occupy 
fault-fissures,  running  either  parallel  or  perpendicular  to  the 
strike  of  the  rocks,  which  is  uniform  over  large  areas;  hence 
a  map  of  the  Cornish  mineral  veins  shows  a  well-marked 
rectangular  arrangement. 

Conformable  and  unconformable  strata.  The  stratified 
rocks  are  laid  down  in  successive  horizontal  sheets,  one  above 
the  other,  so  long  as  the  conditions  remain  uniform.  But 
rock-formation  is  a  slow  process,  and  during  the  course  of 
geological  history  it  has  often  happened  that  the  crust  of  the 
earth  has  been  disturbed,  thus  interrupting  the  regular  succession 
of  strata.  Furthermore,  as  a  result  of  this  disturbance,  the 
strata  may  be  uplifted  and  brought  within  the  influence  of 
agents  of  denudation,  as  described  in  a  later  section,  parts  of 
them  thus  being  destroyed  and  the  raw  edges  of  the  strata 
forming  the  surface  of  the  ground,  or  sinking  again  below  the 
level  of  the  sea.  Subsequently  new  strata  may  be  laid  down 
discordantly  on  these  upturned  edges,  thus  giving  rise  to  what 
is  known  as  an  unconformity,  which  is  really  a  break  in  the 
succession.  There  are  two  chief  types  of  unconformity,  known 
as  overstep  and  overlap  respectively. 

In  the  first  type  the  upper  series  has  a  horizontal  base, 
resting  on  the  edges  of  the  older  upturned  rocks,  which  may  be 
merely  tilted,  or  folded  in  a  variety  of  ways  (see  Fig.  16).  In  an 
unconformity  with  overlap  the  base  of  the  upper  series  is  not 


28 


INTRODUCTION 


[CH. 


horizontal,  but  each  bed  extends  further  in  a  given  direction 
than  the  one  below  (see  Fig.  17).  This  is  the  commonest  type, 
where  deposition  is  taking  place  in  the  sea.  Unconformities 
are  of  the  greatest  possible  importance  in  stratigraphical  geo- 
logy, since  they  indicate  the  periods  of  crust  disturbance  in 
the  earth's  history  and  provide  a  convenient  means  of  dividing 
the  stratified  rocks  into  groups  of  different  ages,  strongly 


o    o 


o 


Fig.  16.     An  unr>onformit3T  with  overstep;   the  upper  horizontal  strata 
rest  on  the  denuded  edges  of  the  lower  folded  strata. 

marked  unconformities  being  often  used  as  the  boundaries  of 
the  rock-systems.  They  indicate  in  fact  the  leading  dates  in 
geological  history,  comparable  to  the  landing  of  Julius  Caesar 
or  the  Norman  conquest  in  English  history. 


Fig.   17.     An  unconformity  with  overlap;    each  bed  of  the  upper 
series  extends  further  to  the  left  than  the  bed  below 

Textures  of  rocks.  In  conformity  with  modern  American 
usage  this  term  is  here  employed  to  designate  those  intimate 
features  of  rocks  which  are  commonly  visible  only  on  a  close 
inspection,  and  are  riot  revealed  by  a  general  survey  of  rock- 
masses  as  a  whole,  or  even  on  a  distant  view,  as  is  the  case  with 
many  of  the  structures  just  described.  The  properties  known 
as  textures  are  always,  or  almost  always,  inherent  in  the  rocks 
from  the  beginning  of  their  existence  in  the  present  form. 
As  might  be  expected  the  textures  of  the  igneous  and  of  the 
sedimentary  rocks  are  fundamentally  different,  and  must  be 
described  separately.  The  textures  of  the  sediments  are 


i]  MINERALS  AND   ROCKS  29 

mostly  dependent  on  the  absolute  and  relative  sizes  of  their 
component  particles,  these  particles  having  been  as  a  rule 
brought  together  by  purely  mechanical  means,  and  subsequently 
cemented  into  a  coherent  mass  by  chemical  processes.  Certain 
rocks  usually  assigned  to  this  class  are  crystalline,  having  been 
formed  by  evaporation  of  solutions,  while  others  are  of  organic 
origin.  The  textures  of  all  these  varieties  will  be  described  in 
later  sections,  treating  of  the  sediments  in  detail. 

The  textures  of  the  igneous  rocks  on  the  other  hand  arise 
directly  from  the  conditions  of  their  crystallization  or  con- 
solidation from  the  fused  state.  They  are  therefore  controlled 
by  the  ordinary  physico-chemical  laws  of  solution.  If  the 
cooling  is  very  rapid  and  under  low  pressure  the  whole  mass 
may  form  a  perfectly  homogeneous  solid  showing  no  differ- 
entiation into  individual  crystals.  This  is  known  as  a  glass, 
and  is  most  common  among  the  volcanic  rocks.  When  the 
cooling  is  sufficiently  slow  the  fused  mass  forms  an  aggregate 
of  crystals  of  various  minerals,  the  nature  of  these  depending 
on  the  original  composition  of  the  material.  The  size  of  the 
crystals  also  is  directly  determined  by  the  rate  of  cooling, 
which  in  most  cases  really  means  the  thickness  of  cover.  The 
size  of  crystals  is  practically  unlimited,  ranging  from  ultra- 
microscopic  dimensions  to  large  individuals,  according  to 
circumstances.  Crystalline  rocks  may  therefore  be  conveniently 
spoken  of  as  cryptocrystalline,  microcrystalline  and  visibly 
crystalline,  the  latter  division  showing  the  greatest  range  of 
size.  Under  special  conditions  the  crystals  of  igneous  rocks 
may  be  very  large  indeed,  being  sometimes  measurable  by 
feet;  such  abnormally  coarse-grained  varieties  are  generally 
called  pegmatite,  the  term  referring  solely  to  size  of  individual 
crystals,  and  not  to  composition.  Pegmatites  generally  occur 
in  the  form  of  veins  and  dykes,  either  cutting  other  igneous 
rocks  or  traversing  sedimentary  or  metamorphic  rocks. 

It  is  very  commonly  the  case  in  the  igneous  rocks  that  the 
crystals  of  one  or  more  minerals  are  conspicuously  larger  than 
the  others,  owing  to  crystallization  in  two  stages  under  different 
conditions.  This  is  known  as  porphyritic  texture,  and  is 
specially  characteristic  of  volcanic  rocks,  though  it  is  also  found 


30  INTRODUCTION  [CH. 

in  the  other  groups.  In  some  cases  well-formed  crystals  which 
have  been  brought  up  from  below  are  embedded  in  a  glassy 
ground-mass  formed  at  the  surface  in  a  lava-flow.  Other  tex- 
tures specially  characteristic  of  volcanic  rocks  are  those  known 
as  vesicular  and  amygdaloidal.  Vesicles  are  simply  hollows  in 
the  rock,  occupied  when  highly  heated  by  bubbles  of  steam  or 
gas,  empty  when  cold.  This  produces  a  spongy  appearance, 
such  as  is  seen  in  pumice-stone,  or  in  artificial  slag  from  a  blast- 
furnace. After  consolidation  of  the  rock  the  vesicles  are  often 
infilled  by  deposits  of  various  minerals  brought  in  by  percolating 
solutions.  The  rock  is  then  said  to  be  amygdaloidal,  from  a 
supposed  resemblance  to  almonds  in  a  cake.  Volcanic  rocks 
also  often  possess  a  streaky  appearance  due  to  flowing  and  rolling 
movements  in  a  viscous  heterogeneous  mass,  and  the  surfaces, 
both  upper  and  under,  of  lava-flows  often  show  a  cindery  or 
slaggy  (scoriaceous)  appearance,  while  the  upper  surfaces  are 
often  ropy  or  corded  owing  to  movement  in  a  viscous  mass. 

The  igneous  rocks.  Although  rocks  of  igneous  origin  must 
be  regarded  as  the  original  source  of  all  the  material  composing 
sedimentary  rocks  and  soils,  they  are  nevertheless  of  but 
subsidiary  importance  to  the  agriculturist,  especially  in  the 
British  Isles,  where  they  occupy  but  a  small  area  of  the  land- 
surface,  and  this  mostly  in  hilly  and  little  cultivated  regions. 
In  other  parts  of  the  world,  e.g.  Canada,  India  and  South  Africa, 
rocks  of  igneous  origin  cover  vast  stretches  of  country,  though 
they  are  often  masked  by  surface  deposits  transported  from 
afar.  Again  in  many  volcanic  districts,  e.g.  Italy,  Java  and 
Central  America,  the  soils  are  largely  formed  by  weathering 
and  decomposition  of  recent  lavas.  Taking  the  world  as  a 
whole  however  there  can  be  no  doubt  that  most  of  the  highly 
cultivated  regions  lie  on  sedimentary  rocks. 

An  igneous  rock  may  be  defined  as  one  which  has  been 
formed  directly  by  consolidation  from  a  state  of  fusion;  an 
instance  of  this  may  be  seen  in  the  solidification  of  a  lava-flow 
emitted  from  a  volcano.  In  this  case  the  cooling  takes  place 
rapidly,  under  atmospheric  pressure  only,  allowing  little  time 
for  the  growth  of  large  crystals.  Under  such  conditions  the 
resulting  rock  may  consist  of  an  aggregate  of  small  crystals  of 


i]  MINERALS   AND   ROCKS  31 

various  minerals,  or  it  may  form  a  completely  homogeneous 
mass  without  crystalline  structure,  which  is  known  to  petro- 
logists  as  a  glass.  Such  are  obsidian,  pitchstone  and  pumice. 

On  the  other  hand  portions  of  molten  material  are  frequently 
injected  into  cavities  formed  in  the  solid  crust  of  the  earth  at 
varying  depths.  When  injected  under  a  thick  cover  of  rock, 
at  great  depths,  the  molten  material  will  naturally  cool  slowly 
and  will  also  be  subjected  to  high  pressure.  These  conditions 
preclude  the  formation  of  glass  and  favour  the  development 
of  large  crystals  of  minerals.  An  example  of  a  rock  formed  under 
such  conditions  is  granite.  As  would  naturally  be  expected, 
small  masses  of  rock  injected  near  the  surface  under  a  thin 
cover,  cool  fairly  quickly  and  therefore  partake  to  some  extent 
of  the  characters  of  both  groups. 

The  chemical  composition  of  rocks  in  general  has  already 
been  described;  it  is  therefore  unnecessary  to  give  here  a  list 
of  the  chemical  constituents  of  the  igneous  rocks,  since  all  the 
constituents  before  enumerated  are  found  in  rocks  of  all  classes. 
The  combination  of  these  constituents  on  cooling  and  crystal- 
lization gives  rise  to  minerals ;  of  the  most  important  of  these 
a  list  has  also  been  given,  and  their  properties  described  (see 

P.  5). 

As  a  result  of  recent  investigations  it  has  been  shown  that 
the  solidification  of  molten  rock-material  takes  place  according 
to  the  laws  of  solutions ;  under  definite  conditions  of  composition, 
temperature  and  pressure,  the  solution  or  magma  will  give  rise 
to  certain  minerals  or  combinations  of  minerals.  In  all  cases 
one  of  the  most  important  controlling  factors  is  the  proportion 
of  silica  in  the  magma,  since  this  governs  the  composition  and 
characters  of  the  minerals  formed.  Most  of  the  minerals  of  the 
igneous  rocks  are  silicates,  i.e.  compounds  of  silica  with  metals, 
or  in  other  words  metallic  salts  of  silicic  acid.  Hence  silica  is 
regarded  as  the  acid  constituent  of  the  magma,  all  others  being 
bases.  A  magma  rich  in  silica  is  called  acid,  and  one  poor  in 
silica  basic,  and  the  same  nomenclature  applies  to  the  rocks 
after  solidification,  whether  crystalline  or  glassy1. 

1  Harker,  The  Natural  History  of  Igneous  Rocks,  London,   1909,  p.   169. 


32  INTRODUCTION  [CH. 

On  the  basis  of  their  percentage  of  silica  the  igneous  rocks 
can  be  divided  arbitrarily  into  four  groups,  as  follows: 

Silica  over  65  per  cent.         ...         ...  Acid. 

Silica  between  65  and  52  per  cent.  Intermediate. 

Silica  between  52  and  45  per  cent.  Basic. 

Silica  below  45  per  cent Ultrabasic. 

These  groups,  though  arbitrary,  correspond  to  real  differ- 
ences of  mineralogical  constitution.  Thus  taking  as  an  example 
the  completely  crystalline  deep-seated  or  plutonic  rocks  the 
general  characteristics  of  each  group  can  be  tabulated  as  follows  : 

,  Acid  group  . . .  Quartz  abundant ;  alkali-felspar  char- 

acteristic, and  dominant  over 
ferromagnesian  silicates. 

Intermediate  group  Quartz  rare  or  absent;  alkali-felspar 
or  plagioclase  dominant  over  ferro- 
magnesian silicates. 

Basic  group  ...  Ferromagnesian  silicates  dominant  over 

felspar;  oxides  of  iron  abundant. 

Ultrabasic  group  . . .  Felspar  absent ;  ferromagnesian  sili- 
cates and  oxides  the  sole  con- 
stituents. 

When  the  composition  of  a  sufficiently  large  number  of 
rock-types  had  been  determined  it  was  found  that  while  the 
acid  and  ultrabasic  groups  were  simple  and  well-marked, 
showing  much  uniformity,  the  intermediate  and  basic  groups 
were  each  clearly  divided  into  two  sub-groups,  characterised 
by  excess  of  potash  and  soda  in  one  case,  and  of  lime  in  the 
other.  These  are  called  the  alkaline  and  subalkaline  groups 
respectively  and  the  full  scheme  of  classification  may  be  best 
expressed  as  in  the  table  below : 

/    basic  alkaline    -  -    intermediate  alkaline    \        . , 
\basic  subalkaline— intermediate  subalkaline/ 

The  igneous  rocks  are  thus  divided  into  six  groups  character- 
ized by  chemical  differences,  to  which  the  mineral  constitution 
of  each  closely  corresponds1. 

1  For  a  full  discussion  of  the  classification  of  the  igneous  rocks  see  Hatch, 
Textbook  of  Petrology,  London,  1914.  In  this  work  will  also  be  found  detailed 
descriptions  of  all  the  important  occurrences  of  such  locks  in  the  British  Isles. 


i]  MINERALS   AND   ROCKS  33 

The  above  diagram  may  now  be  conveniently  translated 
into  the  terms  commonly  employed  by  petrologists  for  these 
natural  groups,  taking  as  an  example,  as  before,  the  plutonic 
rocks : 

. ,     .       /alkali  gabbro — syenite\ 
pendotite  <  , ,  ,.     .       >  granite. 

\      gabbro       — dionte  / 

These  names  are  quite  arbitrary  and  for  the  most  part 
originally  meaningless,  but  they  are  now  accepted  as  implying 
a  certain  limited  range  of  chemical  and  mineralogical  com- 
position. 

Name  %  silica  Essential  minerals 

Granite  ...  Over  65  Quartz,  alkali  felspar  and  mus- 

covite,  biotite,  hornblende  or 
augite. 

Syenite  ...  65-52  Alkali  felspar  and  biotite,  horn- 

blende or  augite,  with  some- 
times nepheline,  leucite  or 
sodalite. 

Diorite  ...  65-52  Plagioclase  felspar,  with  biotite, 

hornblende  or  augite;  often 
some  quartz. 

Alkali  gabbro  52-45  Alkali  felspar  (with  or  without 

plagioclase,  nepheline  or  leu- 
cite)  and  olivine  or  augite  or 
both. 

Gabbro  ...  52-45  Plagioclase  felspar,  with  olivine., 

augite  or  hypersthene. 

Peridotite  ...  Below  45  Exclusively  composed  of  olivine, 

augite,  hypersthene,  horn- 
blende, etc.,  with  iron  ores. 

Note.  Nepheline,  leucite  and  sodalite  are  minerals  allied  to 
the  felspars  in  composition,  but  with  less  silica ;  they  are  only 
found  in  intermediate  and  basic  alkaline  rocks.  Hypersthene 
is  closely  related  to  augite. 

Exactly  similar  considerations  apply  to  the  volcanic  rocks 
formed  from  lavas  extruded  on  the  earth's  surface,  except  that 
in  many  cases  the  presence  of  more  or  less  homogeneous  glassy 
matter  has  to  be  taken  into  account.  Hence  the  mineral 
constitution  of  the-  volcanic  rocks  is  less  definite.  However, 
when  crystalline,  they  contain  the  same  minerals  as  the 

R.  A,  G.  3 


34  INTRODUCTION  [CH. 

corresponding  plutonic   group.     They  may  be  arranged  in  a 
similar  diagram: 

v    i-       -j.    /alkali  basalt — traehytex 
limburgite  <T  ,     .      ;>  rhyolite. 

\       basalt      — andesite/ 

The  rocks  of  the  acid  group  when  completely  glassy  are 
generally  called  obsidian. 

Besides  the  two  well-defined  groups  of  the  plutonic  and 
volcanic  rocks  there  are  as  before  mentioned  a  good  many 
occurrences  of  an  intermediate  type,  rock-masses  partaking  to 
some  extent  of  the  characters  of  both.  These  are  for  the  most 
part  small  masses  that  have  been  intruded  near  the  surface, 
under  a  thin  cover,  so  that  cooling  was  comparatively  rapid, 
though  not  rapid  enough  to  give  rise  to  the  characteristic 
structures  of  volcanic  rocks.  These  are  called  the  Tiypabyssal 
rocks ;  they  occur  for  the  most  part  as  dykes  or  sills ;  they  are 
commonly  in  the  main  crystalline  though  many  varieties  contain 
more  or  less  glass.  This  group  on  the  whole  conforms  also  to 
the  classification  outlined  above,  though  aberrant  forms  are 
rather  numerous ;  the  names  applied  to  the  normal  types  are 
shown  in  the  diagram  below : 

. ,     .      /teschenite —  porphyry  \ 

pendotite  <      -.  ,     .  ,      .      >  quartz  porphyry. 

\  dolente  — porphynte/ 

It  will  be  noticed  that  the  name  here  assigned  to  the  ultra- 
basic  group  is  the  same  as  that  assigned  to  the  corresponding 
subdivision  of  the  plutonic  rocks,  the  reason  being  that  ultrabasic 
rocks  occur  in  small  masses  only  and  it  has  not  yet  been  found 
possible  to  distinguish  the  two  families  satisfactorily. 

The  igneous  rocks  as  soil-formers.  The  function  of  the 
igneous  rocks  as  soil-formers  is  twofold;  in  the  first  place  by 
direct  decomposition  and  weathering  the  minerals  themselves 
form  soil,  which  may  either  remain  in  place  as  a  sedentary 
soil,  or  may  be  transported  elsewhere.  Secondly,  the  igneous 
rocks  yield  materials  for  the  formation  of  sediments,  which  may 
be  afterwards  consolidated  into  rocks,  yielding  soil  by  their 
weathering. 

From  a  study  of  the  facts  detailed  in  the  foregoing  para- 
graphs it  is  evident  that  the  igneous  rocks  vary  much  in  chemical 


i]  MINERALS  AND   ROCKS  35 

and  mineralogical  composition,  hence  the  character  of  the  soils 
formed  from  them  must  vary  also  within  wide  limits.  It  is  of 
course  obvious  that  as  the  acid  rocks  are  rich  in  silica  they  will 
yield  quartz  abundantly,  while  from  the  basic  rocks  this  mineral 
is  absent.  Furthermore  the  acid  rocks  are  richer  in  potash  and 
soda  than  the  basic  rocks,  but  poorer  in  magnesia,  iron  and  lime. 
Again  the  alkali  rocks  are  richer  in  potash  and  soda  and  poorer 
in  lime  than  the  subalkaline  rocks  of  the  same  silica  percentage. 
Of  the  common  constituents  of  rocks,  potash  and  lime  are  of  most 
importance  as  plant  food  and  most  commonly  deficient.  It 
follows  then  that  soils  formed  from  granites  and  syenites  will 
be  rich  in  potash  but  poor  in  lime,  while  soils  formed  from 
diorites  and  gabbros  will  contain  sufficient  lime,  but  will  in  all 
probability  be  deficient  in  potash.  The  precise  composition  of 
the  soil  will  of  course  depend  to  a  large  extent  on  the  kind  of 
weathering  to  which  the  rocks  have  been  subjected,  since 
under  certain  circumstances  soluble  compounds  may  be  formed, 
to  be  subsequently  removed  by  percolating  water  (e.g.  formation 
of  kaolin,  see  p.  52).  But  ordinarily  the  relations  above 
indicated  hold  good  for  potash  and  for  lime.  With  regard  to 
phosphoric  acid,  the  mineral  apatite,  the  ultimate  source  of 
phosphorus  in  nature,  is  almost  equally  abundant  in  all  varieties 
of  igneous  rocks  and  phosphates  are  rarely  deficient  in  soils 
derived  from  them.  The  other  chemical  constituents,  alumina, 
soda,  magnesia  and  iron,  are  generally  present  in  sufficient 
quantity  in  all  soils,  however  formed.  Soils  derived  from 
igneous  rocks  on  the  whole  tend  to  be  rich  in  potash  and  phos- 
phoric acid,  although  these  substances  may  not  always  be 
present  in  an  available  form  in  large  quantity. 

The  actual  processes  of  weathering  and  chemical  decom- 
position by  which  the  minerals  of  the  igneous  and  other  rocks 
are  made  available  in  the  soil  form  a  subject  of  the  greatest 
importance  from  the  agricultural  point  of  view.  They  are 
treated  in  detail  in  Chapter  n. 

The  crystalline  schists.  These  form  a  large  and  varied, 
but  distinctive,  series  of  rocks  belonging  to  the  metamorphic 
group.  Their  origin  is  in  some  instances  still  obscure,  but  the 
majority  of  them  have  certainly  been  formed  by  the  alteration 

3—2 


36  INTRODUCTION  [CH. 

of  normal  igneous  and  sedimentary  rocks.  They  generally 
show  in  a  high  degree  the  properties  of  cleavage,  schistosity 
and  foliation,  as  before  defined.  Many  of  them  also  contain 
certain  peculiar  and  distinctive  minerals,  such  as  are  known  to 
be  developed  under  conditions  of  high  temperature  and  high 
pressure.  Some  of  these  are  also  common  to  the  igneous  rocks, 
while  others  are  mainly  confined  to  this  group.  Most  of  these 
rocks  come  under  the  general  designation  of  gneiss  and  schist, 
but  massive  and  non-foliated  rocks,  such  as  quartzite  and  marble 
are  also  common.  The  true  crystalline  schists  belong  to  the 
earlier  stages  of  the  earth's  history  and  in  most  parts  of  the  world 
they  form  the  foundation  on  which  the  later  rocks  rest.  In 
fact  it  is  still  uncertain  whether  true  crystalline  schists  have 
ever  been  produced  since  the  very  earliest  stages  of  the  earth's 
history. 

Many  of  the  gneisses  are  to  all  intents  and  purposes  quite 
similar  to  the  igneous  rocks,  especially  those  of  the  granite 
group.  The  parallel  structures  so  often  seen  in  them  are  often 
due  to  the  drawing  out  during  flow  of  an  imperfectly  mixed 
liquid  mass  in  process  of  crystallization.  This  gives  a  streaky 
appearance,  with  bands,  often  twisted  and  contorted,  of  varying 
composition  and  colour.  In  other  instances  a  similar  appearance 
is  produced  by  the  recrystallization,  under  intense  pressure,  of 
the  minerals  of  rocks  already  formed.  During  this  process 
much  heat  is  undoubtedly  generated  by  friction,  and  this 
assists  in  the  formation  of  new  minerals,  the  constituents  of  the 
rocks  combining  in  a  different  way.  The  effects  of  the  high 
pressure  and  temperature  is  to  a  large  extent  to  reverse  the 
effects  of  weathering,  and  sediments  that  have  been  highly 
metamorphosed  often  take  on  characters  very  like  those  of  the 
igneous  rocks  from  which  they  were  originally  derived. 

Besides  the  minerals  found  in  the  igneous  rocks,  there  exist 
a  whole  series  of  minerals  specially  characteristic  of  the 
crystalline  schists  and  of  the  metamorphic  rocks  in  general. 
The  most  important  of  these  are  various  silicates  specially  rich 
in  alumina;  such  are  garnet,  andalusite,  kyanite,  sillimanite, 
cordierite  and  staurolite.  In  marbles  formed  from  impure 
limestones  and  dolomites  there  are  found  also  special  minerals 


i]  MINERALS   AND   ROCKS  37 

containing  calcium  and  magnesium,  such  as  wollastonite, 
forsterite,  tremolite  and  spinel.  None  of  these  are  of  much 
importance  as  soil-formers,  and  it  is  unnecessary  to  give  any 
further  account  of  them.  Marbles  are  of  importance  only  in 
so  far  as  they  yield  lime  by  their  decomposition.  The  other 
plant-food  constituents  are  as  a  rule  not  present  in  any  appre- 
ciable quantity.  The  weathering  of  the  true  gneisses  follows 
the  same  lines  as  that  of  the  igneous  rocks;  granitic  gneisses 
yield  soils  rich  in  potash  and  phosphoric  acid,  while  nitrogen 
is  always  deficient.  The  schistose  rocks  formed  from  the  clay- 
like  sediments  form  generally  heavy  soils,  of  very  variable 
character,  and  the  final  result  of  their  weathering  depends  for 
the  most  part  on  the  climatic  conditions,  and  especially  on  the 
amount  and  seasonal  distribution  of  the  rainfall.  If  this  is 
heavy,  most  of  the  soluble  constituents  are  washed  out,  and  the 
result  may  be  a  poor,  hungry  soil,  whereas  in  a  dry  climate 
plant-food  may  be  concentrated  and  irrigation  will  yield  heavy 
crops. 


CHAPTER   II 

WEATHERING 

Introduction.  According  to  the  accepted  definition  the  soil 
is  to  be  regarded  as  the  uppermost  weathered  and  disintegrated 
layer  of  the  earth's  crust ;  its  material  is  derived  either  directly 
or  indirectly  from  rocks,  together  with  a  certain  amount  from 
the  atmosphere.  Any  investigation  of  the  origin  and  characters 
of  the  soil  accordingly  involves  a  knowledge  of  the  processes  of 
disintegration  and  of  chemical  and  mineralogical  change  which 
take  place  in  the  constituents  of  the  rocks.  These  changes  may 
be  summed  up  in  the  general  term  weathering,  since  they  are 
to  a  large  extent  due  to  atmospheric  or  meteoric  agencies. 
The  vital  activities  of  animals  and  plants  also  play  an  important 
part. 

Most  of  the  processes  of  weathering  are  of  a  complex  nature, 
being  frequently  the  result  of  several  causes  acting  simul- 
taneously, and  it  is  often  difficult  to  ascertain  clearly  how  much 
of  the  final  result  is  to  be  attributed  to  each  agent  concerned. 
Hence  a  certain  amount  of  repetition  is  almost  unavoidable. 
A  considerable  degree  of  obscurity  also  prevails  in  some  parts 
of  the  subject,  and  widely  divergent  views  still  continue  to  be 
held  as  to  the  exact  nature  of  the  processes  by  which  certain 
well-known  and  definite  results  are  obtained;  as  an  example 
may  be  mentioned  the  decomposition  of  the  felspars,  and  of 
other  common  silicates.  Much  work  is  still  required  in  this 
field  of  investigation. 

Kinds  of  weathering.  As  a  matter  of  convenience  weathering 
may  be  classified  under  three  general  headings,  namely,  physical, 
chemical  and  organic.  Under  physical  weathering  are  included 


CH.  n]  WEATHERING  39 

all  those  processes  resulting  in  a  change  in  the  state  of  aggrega- 
tion of  the  mineral  particles  of  a  rock,  unaccompanied  by  any 
chemical  or  mineralogical  alteration.  The  rocks  are  thus 
broken  up  into  smaller  portions  or  even  into  their  constituent 
mineral  grains.  This  in  itself  promotes  the  state  of  fine  division 
which  is  necessary  in  a  soil,  and  by  increasing  the  free  surface 
area  of  the  particles  it  also  facilitates  the  action  of  chemical 
weathering  agents.  The  second  group  includes  all  those  changes- 
resulting  in  an  alteration  of  the  composition,  not  only  of  the 
rock  as  a  whole,  but  also  of  individual  particles  of  it.  This 
necessarily  involves  mineralogical  changes  which  are  often  far- 
reaching  in  their  character.  One  of  the  most  important  results 
of  chemical  weathering  is,  in  a  general  way,  to  render  the 
supplies  of  plant-food  contained  in  the  rocks  more  readily 
available  for  assimilation  by  the  plant,  thus  increasing  the 
fertility  of  the  soil.  Chemical  weathering  also  produces 
exceedingly  important  results  by  leading  directly  to  disintegra- 
tion and  subdivision  of  the  soil  particles,  often  accompanied 
by  removal  of  certain  constituents  in  solution  or  in  suspension. 
The  effects  of  the  vital  activity  of  animals  and  plants  are  partly 
physical,  assisting  disintegration,  and  partly  of  a  chemical 
nature.  The  latter  class  of  changes  are  often  highly  complex, 
involving  the  formation  of  various  chemical  compounds  whose 
exact  character  is  still  obscure.  In  particular,  the  action  of 
bacteria  and  other  protozoa  is  undoubtedly  of  great  importance 
in  soil  formation,  but  on  this  subject  the  information  at  our 
disposal  is  scanty  and  somewhat  contradictory.  It  is  well 
known  that  bacteria  play  an  important  part  in  certain  processes 
in  the  soil,  especially  in  connexion  with  the  formation  of 
nitrates,  but  it  is  highly  probable  that  they  have  an  important 
influence  also  in  many  other  ways,  which  still  remain  to  be 
worked  out  in  detail1. 

Climate  and  weathering.  Among  the  most  important 
results  of  recent  investigations  "into  the  origin  and  characters  of 
soils,  carried  out  for  the  most  part  in  Germany  and  Russia,  has 
been  the  recognition  of  the  great  influence  of  climatic  conditions 
in  determining  the  ultimate  character  of  the  soil.  Since 

1  Russell,  The  Fertility  of  the  Soil,  Cambridge,  1913,  Chapters  I  and 


40  WEATHERING  [CH. 

physical  and  chemical  processes  are  controlled  to  a  great  extent 
by  temperature  and  the  presence  or  absence  of  water,  and  since 
these  are  also  important  factors  in  climate,  it  follows  that  the 
type  of  weathering  in  any  given  area  must  also  be  controlled 
by  these  factors. 

Owing  to  meteorological  causes,  into  which  it  is  unnecessary 
to  enter  here,  the  whole  earth  can  be  divided  into  seven 
climatic  zones  or  belts,  as  follows  : 

The  equatorial  or  tropical  zone,  on  either  side  of  the  equator, 
characterized  by  high  temperature  and  heavy  rainfall,  leading 
to  luxuriant  vegetation,  which  tends  to  facilitate  decomposition 
of  the  rocks  by  chemical  and  organic  agencies;  owing  to  the 
protective  effect  of  thick  vegetation  transport  is  in  abeyance, 
and  the  soils  are  deep  and  rich  in  plant-food.  The  action  of 
bacteria,  being  favoured  by  the  climate,  is  also  at  a  maximum. 

The  desert  zones.  On  either  side  of  the  equatorial  belt  in 
each  hemisphere  is  a  region  of  varying  width  characterized  by 
high  temperature  and  very  small  precipitation,  forming  the 
great  rainless  regions  of  the  world.  Here  water  action  is  in 
abeyance,  while  animals  and  plants  are  almost  completely 
absent. 

The  temperate  zones  are  regions  of  moderate  temperature 
and  as  a  rule  of  fairly  abundant  rainfall,  though  in  this  respect 
much  local  variation  exists.  The  greater  part  of  the  land- 
surface  is  covered  by  vegetation  and  animals  are  numerous. 
The  types  of  weathering  which  prevail  in  these  zones  are  of  a 
mixed  character  and  all  the  ordinary  geological  agents  are 
operative  to  a  greater  or  less  extent.  The  seasonal  variations 
of  climate  are  strongly  marked,  and  at  different  times  of  the 
year  different  agents  of  weathering  are  dominant.  The  water- 
systerns  of  the  land  are  well  developed,  and  transport  of  material 
by  running  water  plays  an  important  part  in  soil-formation. 
Weathering  and  transport  are  as  a  rule  nicely  balanced  and 
weathered  material  does  not  commonly  accumulate  in  place 
to  great  depths,  as  in  tropical  regions. 

The  arctic  zones  are  characterized  by  extremely  low  tempera- 
ture, animal  and  vegetable  life  is  scanty,  and  soil-formation  is 
at  a  minimum.  The  most  important  geological  agent  is  frost. 


n]  WEATHERING  41 

and  the  low  temperature  is  unfavourable  to  chemical  weathering. 
Large  surfaces  are  covered  by  permanent  ice  and  snow,  and  bare 
rock  is  abundant.  The  absence  of  light  for  part  of  the  year  is 
unfavourable  to  plant  growth,  and  in  truly  arctic  regions 
agriculture  is  non-existent.  The  actual  precipitation  is  generally 
small  and  the  low  temperature  keeps  most  of  the  water  in  the 
solid  form;  hence  for  practical  purposes  the  climate  is  dry. 
The  climatic  conditions  of  these  zones  can  be  conveniently 
summarized  in  a  tabular  form,  as  follows: 

Tropical  zone  ...  Hot  and  damp. 

Desert  zones  ...  Hot  and  dry. 

Temperate  zones      ...  Cool  and  damp. 

Arctic  zones Cold  and  dry. 

These  climatic  zones  are  well  marked  and  on  the  whole  they 
follow  the  parallels  of  latitude,  with  variations  induced  by  the 
relative  positions  of  land  and  sea.  An  important  factor  in  the 
latter  consideration  is  the  general  north-and-south  trend  of  the 
continents  and  oceans.  Stated  in  general  terms  the  central 
parts  of  the  continents  tend  to  be  drier  than  the  mean,  with 
greater  variations  of  temperature,  i.e.  hotter  summers  and 
colder  winters,  while  over  the  oceans  the  conditions  are  more 
equable.  Prevailing  winds  and  ocean  currents  also  have  an 
important  influence.  For  example,  Western  Europe  is  much 
warmer  in  winter  than  corresponding  latitudes  in  Eastern 
America  or  Eastern  Asia.  Cultivation  can  consequently  be 
carried  on  in  Norway  within  the  Arctic  circle,  while  Labrador, 
in  the  latitude  of  Britain,  is  a  barren  waste.  Again  the  south- 
ward-flowing Mozambique  current  makes  the  climate  of  Natal 
almost  tropical.  Owing  to  deficiency  of  rainfall  the  interior 
of  Asia  is  largely  desert,  and  indeed  in  this  region  the  temperate 
zone  is  practically  absent;  the  desert  region  extending  north- 
wards till  it  meets  the  frozen  swamps  of  Eastern  Siberia1. 

Of  almost  equal  importance  with  latitude  as  a  climatic 
influence  is  height  above  sea-level.  In  the  mountain  ranges 
of  the  temperate  zone  and  in  the  highest  mountains  of  the 

1  For  a  good  account  of  the  general  distribution  of  types  of  climate  see 
Lake,  Physical  Geography,  Cambridge,  1915. 


42  WEATHERING  [CH. 

tropics  the  climatic  and  geological  conditions  resemble  those  of 
the  arctic  zones;  glaciers  abound  in  many  mountains  in  low 
latitudes,  e.g.  the  Alps,  the  Himalaya,  the  mountains  of  east- 
central  Africa,  the  Andes,  the  mountains  of  New  Zealand  and 
others.  In  fact  everywhere  above  the  permanent  snow-line, 
at  whatever  height  that  may  happen  to  be,  the  conditions  are 
arctic.  The  presence  or  absence  of  water  is  also  of  great 
importance,  as  witness  the  fertility  of  the  oases  of  the  Sahara 
in  the  desert  zone,  and  of  certain  arid  regions  in  South  Africa 
and  Australia,  when  artificially  irrigated. 

Weathering  by  chemical  processes.  Under  this  heading  are 
included  all  those  processes  in  rocks  that  lead  to  disintegration 
and  alteration  by  chemical  means.  The  chemical  processes 
which  operate  in  this  way  are  numerous  and  often  complex  in 
their  action.  The  following  list  comprises  the  most  important 
of  them: 

1.  Solution. 

2.  Hydration. 

3.  Hydrolysis. 

4.  Pneumatolysis. 

5.  Oxidation. 

6.  Reduction. 

7.  Carbonation. 

Nearly  all  the  chemical  processes  which  take  place  in 
minerals  and  rocks  can  be  ranged  under  one  or  other  of  these 
heads;  it  must  however  always  be  remembered  that  two  or 
more  of  these  processes  may  be,  and  generally  are,  operative 
at  the  same  time,  and  it  is  often  a  matter  of  great  difficulty  to 
ascertain  to  which  class  or  classes  a  given  case  should  be  assigned. 
Thus  for  example  solution,  hydration  and  hydrolysis  are  very 
closely  allied,  while  oxidation  and  nearly  all  ordinary  chemical 
reactions  are  greatly  facilitated  by  the  presence  of  water. 
A  familiar  example  is  the  rusting  of  iron ;  indeed  many  chemical 
changes  cannot  take  place  at  all  if  the  substances  are  perfectly 
dry.  Again,  no  natural  waters  are  absolutely  pure,  and  the 
gases  and  solids  dissolved  in  the  water  play  an  important  part 
in  chemical  processes  of  weathering. 


n]  WEATHERING  43 

Solution.  The  simplest  of  all  weathering  processes  is 
solution.  Most  minerals  are  soluble  to  a  certain  extent  in 
pure  water,  though  the  degree  of  solubility  is  in  general  very 
small.  Some  of  the  salts  of  sodium,  potassium  and  magnesium, 
especially  the  chlorides  and  sulphates,  are  however  readily 
soluble.  If  beds  containing  these  salts  are  leached  out  by 
percolating  water,  a  general  subsidence  and  collapse  of  overlying 
strata  may  take  place,  possibly  forming  lake-basins.  With  the 
exception  of  rock-salt,  these  minerals  are  very  rare  in  large 
masses.  However  they  certainly  exist  largely  in  a  finely 
divided  state  in  the  soils  of  many  regions  and  their  removal  by 
solution  has  an  important  bearing  on  the  fertility  of  the  soil. 
Similar  considerations  apply  in  a  less  degree  to  calcium  sulphate, 
which  exists  in  two  natural  forms,  called  anhydrite  and  gypsum 
respectively. 

The  different  forms  of  calcium  carbonate  and  the  mineral 
dolomite  are  also  comparatively  soluble  in  water  and  their 
removal  in  solution  has  very  important  effects  on  the  denudation 
of  limestone  regions.  This  subject  will  be  treated  later  under 
the  heading  of  carbonation.  •  Some  of  the  compounds  of  iron 
are  easily  soluble  in  water,  e.g.  the  chlorides  and  sulphates,  and  if 
present  in  rocks  or  soils  they  are  liable  to  be  removed.  The 
solubility  of  silicate  minerals  is  universally  very  small  and  in 
fact  the  effect  of  pure  water  on  most  of  them  is  negligible  at  the 
atmospheric  temperature  and  pressure.  But  since  the  solubility 
of  nearly  all  minerals  is  increased  by  rise  of  temperature  and 
pressure  it  follows  that  at  great  depths  within  the  earth  solution 
must  be  more  active.  This  is  shown  by  the  fact  that  hot 
springs  coming  from  great  depths  contain  much  mineral  matter 
derived  from  the  rocks  through  which  the  water  has  passed 
(see  pp.  46  and  52). 

Hydration.  This  process  is  of  very  common  occurrence 
in  nature  and  has  an  important  influence  on  soil-formation. 
Most  of  the  chemical  elements  have  the  power  of  forming 
hydroxides  in  the  presence  of  water,  and  some  do  so  with  great 
readiness.  A  familiar  example  of  the  process  is  the  slaking  of 
quicklime,  which  is  represented  by  the  equation: 


44  WEATHERING  [CH. 

The  slaking  of  lime  is  accompanied  by  evolution  of  heat 
and  disintegration  or  crumbling  of  the  lime.  Processes  very 
similar  to  this,  but  much  less  rapid  in  their  action,  occur  in  the 
natural  disintegration  of  rocks,  and  lead  to  important  results. 
Oxides  of  iron  are  specially  liable  to  hydration ;  thus  haematite, 
Fe203,  is  hydrated  to  form  limonite,  2Fe203  .  3H20,  and  hydrates 
of  iron  with  different  proportions  of  water  exist  as  natural 
minerals.  Hydrates  of  alumina,  such  as  bauxite  and  diaspore, 
also  commonly  occur  in  weathered  rocks. 

Many  minerals  also  are  known  consisting  of  basic  salts  of 
certain  metals,  while  others  possess  water  of  crystallization. 
A  good  example  of  the  latter  is  the  mineral  gypsum,  which  has 
the  formula  CaS04  .  2H20 ,  while  calcium  sulphate  also  exists 
naturally  as  the  anhydrous  mineral  anhydrite,  CaS04.  The 
formation  of  gypsum  from  anhydrite,  a  common  process, 
results  in  an  increase  of  volume  by  about  33  per  cent.,  so  that 
it  has  a  powerful  disintegrating  effect. 

Hydrated  silicates  are  also  exceedingly  common,  and  their 
constitution  is  generally  very  complex.  Such  are  the  great 
group  of  the  zeolite  minerals  which  are  supposed  by  some 
authorities  to  exist  largely  in  soils,  though  this  is  still  somewhat 
doubtful.  It  is  highly  probable  that  the  formation  of  many 
hydrated  silicates  is  really  due  to  hydrolysis,  as  will  be  explained 
in  the  next  section.  The  formation  of  chlorite  from  biotite 
may  perhaps  be  a  case  of  simple  hydration ;  it  is  very  common 
in  the  weathering  of  granites ;  it  is  possible  however  that  there 
is  here  in  addition  some  removal  of  potash  in  solution. 

Hydrolysis.  When  a  salt  of  a  strong  base  and  weak  acid, 
for  example,  sodium  carbonate,  is  dissolved  in  water,  disso- 
ciation occurs  and  the  solution  possesses  an  alkaline  reaction. 
This  is  due  to  a  decomposition  which  may  be  represented  by 
the  following  equation: 

Na2C03  +  2H20  =  2NaOH  +  H2C03. 

The  alkali  felspars  may  also  be  regarded  as  compounds  of 
strong  bases,  K20  and  Na20,  with  a  weak  acid,  aluminosilicic 
acid,  and  in  contact  with  water  a  similar  reaction  also  takes 
place,  though  with  extreme  slowness.  In  the  case  of  orthoclase 


n]  WEATHERING  45 

this  results  in  the  formation  of  kaolinite  or  its  amorphous 
equivalent,  halloysite;  the  change  may  be  represented  by  the 
equation : 

K20 .  A1203 .  6Si02  -f  3H20  =  2KOH+ A1203 .  2Si02 .  2H20  +  4Si02. 

The  colloidal  silica  thus  set  free  is  soluble  in  caustic  potash 
and  is  removed  in  solution,  thus  leaving  a  kaolinitic  residue, 
which  is  an  important  constituent  of  many  soils  and  in  its 
purest  form  constitutes  china-clay. 

Similar  reactions  occur  when  any  of  the  common  rock- 
forming  silicates  of  magnesia,  lime,  etc.,  are  acted  on  by  water, 
resulting  in  the  formation  of  hydrated  and  free  silica,  together 
with,  in  some  cases,  hydrated  silicates  of  these  bases,  which 
appear  to  be  often  of  rather  complex  composition.  Thus  for 
example  the  formation  of  serpentine  from  olivine  is  probably 
to  be  attributed  to  hydrolysis,  serpentine  being  a  hydrated 
silicate  of  magnesia. 

Pneumatolysis.  This  term  is  used  to  signify  the  action  of 
highly  heated  and  therefore  chemically  active  gases  on  rocks 
and  minerals.  Its  practical  effect  in  weathering  and  soil- 
formation  is  local  and  limited,  being  manifested  chiefly  in 
volcanic  regions  and  districts  where  hot  springs  are  prevalent. 
The  pneumatolytic  decomposition  of  minerals  is  due  partly  to 
highly  heated  water- vapour  and  partly  to  such  volatile  elements 
as  boron  and  fluorine.  It  is  specially  prevalent  in  connexion 
with  granite  intrusions,  this  rock  being  often  weathered  and 
shattered  down  to  great  depths.  A  good  example  is  afforded 
by  the  formation  of  kaolinite  from  the  granites  of  Devon  and 
Cornwall,  Karlsbad,  etc.  This  process  is  essentially  the  same 
as  hydrolysis,  but  carried  on  at  a  higher  temperature  and 
pressure  and  therefore  more  active.  Perhaps  the  commonest 
mineral  product  of  pneumatolysis  in  granites  is  tourmaline, 
which  is  very  stable,  and  in  small  quantities  a  common  con- 
stituent of  sands  and  soils.  The  pneumatolytic  action  of 
steam  and  other  vapours  doubtless  accounts  in  part  for  the 
rapid  weathering  and  disintegration  of  lava-flows,  which  in  a 
comparatively  short  time  often  yield  soils  of  extraordinary 
fertility.  In  regions  of  expiring  volcanic  activity  the  ground 


46  WEATHERING  [CH. 

is  often  at  a  high  temperature  at  a  small  depth  from  the  surface, 
as  well  as  being  largely  penetrated  by  heated  waters ;  in  such 
places  decomposition  of  rocks  and  minerals  is  rapid  and  far- 
reaching,  most  of  the  common  silicate  minerals  being  here 
almost  completely  decomposed,  and  the  rocks  much  dis- 
integrated. In  such  regions  hot  springs  often  bring  to  the 
surface  much  dissolved  matter,  especially  carbonate  of  lime, 
silica  and  various  salts  of  potassium,  sodium  and  magnesium 
(mineral  springs). 

Oxidation.  This  is  undoubtedly  a  most  important  process 
in  weathering  and  soil-formation,  but  its  effects  are  always  so 
closely  associated  with  hydration  and  other  chemical  processes 
that  it  is  very  difficult  to  give  any  clear  and  connected  account 
of  them.  Oxidation  is  most  conspicuous  in  the  case  of  iron 
compounds,  since  the  change  from  the  ferrous  to  the  ferric 
state  generally  involves  a  notable  alteration  of  colour.  Ferrous 
compounds  are  generally  black,  green  or  grey,  whereas  ferric 
compounds  usually  show  some  shade  of  yellow,  brown  or  red. 
This  accounts  for  the  commonly  observed  fact  that  the  soils 
overlying  dark-coloured  rocks,  such  as  basalt,  slate  or  grey  clay, 
have  a  colour  quite  different  from  that  of  the  unaltered  rock, 
and  in  the  same  way  rocks  largely  composed  of  black  or  green 
silicates  containing  iron  always  have  a  brown  weathered  crust. 
It  can  often  be  observed  in  deep  mines  that  this  change  of  colour 
has  extended  down  to  depths  measurable  by  hundreds  of  feet, 
in  fact,  as  far  down  as  the  rocks  are  saturated  with  water 
coming  from  the  surface,  carrying  with  it  oxidizing  agents,  and 
especially  atmospheric  oxygen.  Although  oxidation  is  most 
readily  observable  in  iron  compounds,  it  also  occurs  in  many 
other  cases. 

Oxidation  always  involves  a  change  of  volume  in  the  minerals 
affected  and  thus  produces  mechanical  stresses,  leading  to 
disintegration.  Many  highly  oxidized  substances,  such  as  ferric 
compounds,  crystallize  with  difficulty  under  normal  conditions 
and  tend  to  remain  in  a  granular  and  amorphous  form,  thus 
facilitating  the  formation  of  a  fine  tilth. 

Oxidation  also  plays  a  most  important  part  in  the  decom- 
position and  decay  of  organic  matter,  the  ultimate  products 


n]  WEATHERING  47 

being  carbon  dioxide  and  water;  these  processes  are  most 
commonly  brought  about  by  means  of  bacteria  and  are  highly 
complex  in  their  nature.  The  complex  nitrogenous  compounds 
occurring  in  animal  and  vegetable  tissues  are  converted  by  a 
series  of  bacterial  changes  into  nitric  acid  and  nitrates,  which 
are  of  immense  importance  as  sources  of  plant  food.  The 
nature  of  the  organic  constituents  of  the  soil  will  be  considered 
again  in  a  later  section. 

Reduction.  This  process,  which  is  the  converse  of  oxidation, 
is  not  of  much  significance  in  weathering  and  soil-formation, 
since  the  prevailing  conditions  are  generally  unfavourable  to 
it.  Reduction  does  take  place  however  in  sour  water-logged 
soils,  where  the  oxygen  of  the  air  is  excluded  and  anaerobic 
bacteria  flourish.  It  is  on  the  other  hand  the  prevailing  reaction 
in  the  formation  of  sedimentary  deposits  in  deep  water  and 
especially  in  the  sea,  where  ferric  compounds  are  reduced  to  the 
ferrous  state,  resulting  in  the  formation  of  iron  sulphides, 
either  the  black  sulphide,  FeS,  or  pyrites  and  marcasite,  FeS2. 
To  the  presence  of  these  substances  is  largely  due  the  generally 
prevailing  blue  or  grey  colour  of  unweathered  clays.  This 
reduction  is  chiefly  brought  about  by  the  decomposition  of 
organic  matter  buried  in  the  sediment,  the  remains  of  animals 
and  plants  that  lived  in  the  sea  or  were  carried  down  from  the 
land.  It  is  from  this  decay  also  that  the  sulphur  is  derived 
for  the  formation  of  the  sulphides. 

Carbonation.  This  term  is  generally  employed  to  designate 
the  effect  on  rocks  and  minerals  of  carbon  dioxide  dissolved 
in  water.  This  is  undoubtedly  a  most  important  agent  in 
weathering  and  denudation,  and  until  recently  almost  all  the 
chemical  alterations  in  minerals  were  attributed  to  it.  Of  late 
years  however,  the  idea  has  begun  to  gain  ground  that  some  of 
these  changes  can  be  explained  in  other  ways,  e.g.,  by  hydrolysis 
(see  p.  44). 

Carbon  dioxide  when  dissolved  in  water  forms  an  acid, 
carbonic  acid,  which  has  however  never  been  isolated  in  the 
pure  state.  The  solubility  of  carbon  dioxide  is  increased  by 
pressure,  hence  natural  waters  at  great  depths  must  have  a 
more  powerful  effect  than  those  at  and  near  the  surface.  Under 


48  WEATHERING  [CH. 

the  most  favourable  circumstances  however  carbonic  acid  is 
a  weak  acid,  and  its  effects  in  displacing  other  acid  radicles  only 
become  noticeable  when  continued  for  a  considerable  time. 

The  most  conspicuous  and  easily  observed  effect  of  dissolved 
carbon  dioxide  is  its  action  on  calcium  carbonate.  When  these 
substances  are  brought  in  contact  a  special  reaction  occurs  and 
a  bicarbonate  is  formed,  thus : 

CaC03  +  C02  +  H20  =  CaH2(C03)2. 

This  compound  is  more  soluble  than  the  normal  carbonate, 
and  it  is  in  this  form  that  calcium  carbonate  really  exists  in 
natural  waters.  It  is  unstable  and  is  broken  up  by  heat,  or 
release  of  pressure,  again  forming  normal  carbonate  and  carbon 
dioxide.  For  this  reason  calcareous  waters  from  deep-seated 
springs  often  deposit  much  carbonate  of  lime  when  they  reach 
the  surface. 

The  solvent  action  of  carbonated  water  plays  a  very 
important  part  in  the  denudation  of  limestone  regions,  and  in 
the  weathering  and  disintegration  of  rocks  containing  carbonates, 
either  as  their  principal  constituents  or  as  cements,  e.g.  calcareous 
sandstones.  In  the  latter  case  the  cement  is  removed  and  the 
grains  of  quartz  and  other  minerals  are  left  free  and  may  form 
sandy  residual  deposits.  In  a  similar  way  magnesium  and  iron 
compounds  may  be  removed  in  solution  as  carbonates,  being 
somewhat  soluble  in  that  form,  while  the  carbonates  of  sodium 
and  potassium  are  readily  soluble  salts ;  these  latter  are  found 
in  considerable  quantity  in  the  waters  of  certain  salt  lakes, 
having  been  carried  into  them  by  streams;  hence  it  follows 
that  they  must  be  present  in  river  waters  and  in  natural  under- 
ground waters  generally. 

Weathering  by  physical  processes.  The  agents  concerned  in 
physical  weathering  are  almost  entirely  atmospheric  in  their 
origin,  consequently  they  are  for  the  most  part  strictly 
dependent  on  climate.  The  dominant  factors  are  variations  of 
temperature  and  presence  or  absence  of  moisture.  In  the  first 
place  it  is  obvious  that  many  of  the  chemical  changes  above 
described  must  also  have  an  important  mechanical  effect. 
Thus  solution  of  the  binding  material  of  a  cemented  rock  causes 


n]  WEATHERING  49 

disintegration  of  the  rock  as  a  whole,  the  insoluble  particles 
being  set  free.  This  action  alone  is  of  itself  sufficient  in  some 
cases  to  form  a  residual  soil.  Hydration  and  oxidation  also 
bring  about  disintegration  by  change  of  volume,  and  other 
instances  might  also  be  cited.  These  however  are,  but  subsi- 
diary effects.  Among  purely  mechanical  agencies  of  weathering 
the  most  important  are  expansion  and  contraction  consequent 
on  changes  of  temperature,  and  the  expansion  of  water  in 
freezing. 

It  is  well  known  that  all  natural  substances  expand  and 
contract  in  volume  as  the  temperature  changes.  In  nearly  every 
case  expansion  is  caused  by  rise  of  temperature,  although  the 
absolute  amount,  or  coefficient  of  expansion,  varies  in  different 
substances.  Again  in  crystalline  minerals  change  of  volume  is 
a  vectorial  property,  having  different  values  in  different  direc- 
tions. Since  rocks  are  not  as  a  rule  homogeneous,  but  consist 
of  an  aggregate  of  different  minerals,  the  expansion  is  usually 
unequal  and  differential  strains  are  set  up,  leading  to  fracture 
and  shattering  of  the  rock  as  a  whole,  or  of  the  individual 
crystals.  It  is  evident  that  the  more  sudden  the  change  of 
temperature,  the  greater  will  be  the  strains,  and  the  greater 
the  consequent  shattering. 

The  conditions  most  favourable  to  a  full  development  of 
this  process  are  to  be  found  in  arid  regions  in  low  latitudes, 
in  other  words,  in  the  desert  zones.  Here  the  temperature  by 
day  is  extremely  high,  while  after  sunset  owing  to  the  clear 
sky  radiation  is  strong  and  the  fall  of  temperature  is  very 
sudden.  In  consequence  of  this  change  the  rocks  are  cracked 
and  shattered  to  a  great  extent.  From  vertical  faces  and  on 
steep  slopes  the  loosened  fragments  fall  down  and  form  great 
screes  at  the  foot,  but  on  level  surfaces  the  effect  is  limited. 
Rocks  are  bad  conductors  and  a  coating  of  loosened  fragments 
of  comparatively  small  depth  will  prevent  heating  and  cooling 
of  the  underlying  solid  rock.  Hence  in  hot,  arid  regions  the 
hills  will  be  worn  away  much  quicker  than  the  plains,  tending 
towards  a  general  levelling  of  the  surface.  The  actual  daily 
variations  of  temperature  in  such  regions  are  often  very  great, 
amounting  sometimes  to  70°  F.,  and  blocks  weighing  up  to 

R.  A.G.  4 


50  WEATHERING  fen. 

several  hundred  pounds  are  often  broken  off:  with  a  loud  report. 
But  more  commonly  quite  thin  scaly  layers  break  off  from  the 
surface,  and  such  scaly  weathering,  the  desquamation  of 
Richthofen1,  is  specially  liable  to  occur  in  granitic  rocks.  In 
temperate  climates,  such  as  the  British  Isles,  this  kind  of 
physical  weathering  is  probably  of  minor  importance,  but  in 
certain  parts  of  the  United  States,  in  Texas  and  California,  and 
even  in  Massachusetts,  it  is  very  prominent2. 

In  temperate  and  cold  climates  the  most  important  physical 
agent  of  disintegration  is  the  expansion  of  water  on  freezing. 
When  the  rainfall  is  abundant  the  crevices  and  pores  of  rocks 
are  filled  with  water  and  this  on  freezing  expands  and  exerts 
an  almost  irresistible  force.  The  pressure  exerted  in  this  way 
is  more  than  sufficient  to  overcome  the  cohesion  of  all  ordinary 
rocks.  The  expansion  is  very  considerable;  1  c.c.  of  water  at 
0°  C.  when  frozen  yields  1-0907  c.c.  of  ice;  the  increase  of 
volume  is  therefore  about  -fa  of  the  whole.  When  the  tem- 
perature falls  below  0°  C.  the  water  in  the  rocks  freezes,  expands 
and  shatters  them,  but  it  is  only  after  a  thaw  that  the  effects 
become  manifest,  since  while  the  water  is  frozen  it  welds  the 
whole  into  a  coherent  mass,  which  falls  to  pieces  when  the  ice 
melts.  The  process  is  well  illustrated  by  the  spongy  state  of 
an  ordinary  road  or  gravel  path  after  a  thaw.  When  the 
water  freezes  the  whole  surface  rises  bodily,  but  is  hard  so  long 
as  it  is  frozen;  only  when  the  ice  disappears  is  the  increased 
distance  apart  of  the  stones  perceptible. 

In  cold-temperate  and  arctic  regions  the  shattering  effects 
of  frost  play  a  similar  part  to  changes  of  temperature  in  the 
tropics.  Blocks  of  rock  of  all  sizes  are  broken  off,  forming 
screes  among  mountains,  and  on  level  ground  covering  the 
surface  with  a  layer  of  disintegrated  rock-debris.  This  action 
is  of  great  importance  in  soil-formation,  and  on  it  depend  the 
beneficial  effects  of  autumn  and  winter  ploughing,  especially 
on  heavy  clay  land.  The  blocks  of  soil  turned  up  by  the  plough 
in  the  autumn  are  shattered  by  the  frosts  of  winter,  thus 

1  Richthofen,  Fuhrer  fur  Forschungsreisende,  Berlin,  1886,  p.  94.     Walther, 
Das  Gesetz  der   Wustenbildung,  Berlin,  1900. 

2  Merrill,  Rocks,  Rock-weathering  and  Soils,  New  York,  1897,  p.  181. 


ii]  WEATHERING  51 

facilitating  the  formation  of  a  fine  tilth  at  spring  seed-time. 
The  thorough  aeration  thus  brought  about  also  assists  to  render 
available  the  plant  food  in  the  soil,  by  means  of  oxidation, 
carbonation  and  the  activity  of  bacteria.  In  high  mountain 
regions  of  the  temperate  zone  and  at  all  elevations  in  the  arctic 
regions  this  is  by  far  the  most  important  agent  of  disintegration, 
since  at  low  temperatures  many  chemical  processes  are  in 
abeyance,  and  micro-organisms  are  unable  to  exist.  Most 
chemical  weathering  depends  directly  or  indirectly  on  the 
presence  of  water,  and  when  the  water  is  frozen  the  weathering 
processes  cannot  go  on. 

Although  wind  can  scarcely  be  considered  as  a  weathering 
agent  in  the  ordinary  sense,  yet  it  does  possess  a  certain  power 
of  disintegration  through  the  kinetic  energy  of  transported 
particles.  This  may  be  compared  to  the  sand-blast  as  employed 
for  etching  glass  and  other  hard  substances.  This  natural 
sand-blast,  when  long  continued,  must  have  some  effect  in 
producing  very  finely  comminuted  material,  and  in  fact  the 
fine  dust  of  the  deserts  is  well  known ;  it  is  probably  produced 
largely  in  this  way,  partly  by  friction  of  loose  particles  against 
each  other  and  partly  by  their  grinding  effect  on  solid  rocks. 
Soft  minerals  and  those  possessing  a  good  cleavage  are  generally 
absent  from  wind-blown  sands,  having  been  reduced  to  impalp- 
able particles  by  long-continued  attrition. 

The  weathering  of  some  common  rock-forming  minerals. 
The  operation  of  the  ordinary  processes  of  weathering  can 
perhaps  be  most  satisfactorily  illustrated  by  a  consideration  of 
the  chemical  and  mineralogical  changes  taking  place  in  the 
case  of  some  of  the  commoner  minerals  constituting  the  greater 
part  of  the  rocks  of  the  earth's  crust.  In  nearly  all  cases  these 
changes  are  the  resultant  of  several  processes  acting  at  once. 
It  is  comparatively  seldom  that  they  are  perfectly  simple  in 
character  and  referable  to  the  influence  of  one  agent  alone. 

Quartz  is  one  of  the  most  stable  of  all  minerals ;  at  ordinary 
temperatures  and  pressures  it  is  unaffected  by  any  of  the 
ordinary  chemical  reagents,  and  is  only  effectively  attacked 
by  hydrofluoric  acid.  Consequently  it  undergoes  no  chemical 
changes,  and  therefore  tends  to  be  concentrated  in  both  residual 

4—2 


52  WEATHERING  [CH. 

and  transported  deposits.  The  only  change  which  in  practice 
takes  place  in  particles  of  quartz  is  a  gradual  reduction  in  size, 
accompanied  by  rounding,  arising  from  mutual  attrition  and 
occasional  contact  with  still  harder  mineral  particles.  Hence 
quartz  is  without  doubt  the  commonest  of  all  minerals  in 
sedimentary  rocks  and  in  soils.  It  is  the  principal  constituent 
of  sands,  and  is  also  abundant  in  muds  and  other  fine-grained 
clastic  deposits.  Grains  of  quartz  may  pass  on  from  one 
stratified  deposit  to  another  with  little  or  no  change. 

The  cryptocrystalline  and  non-crystalline  forms  of  silica  are 
more  easily  dissolved  than  quartz,  colloidal  silica  being  fairly 
soluble  in  alkaline  solutions ;  colloidal  silica  is  a  by-product  of 
the  decomposition  of  many  silicates,  as  will  appear  in  detail 
hereafter. 

It  has  however  been  shown  experimentally  by  Pfaff  and 
others  that  under  increased  pressures  and  at  high  temperatures 
even  quartz  is  appreciably  dissolved  by  water,  though  but  to 
a  small  extent.  It  is  clear  that  the  highly  superheated  water 
occurring  at  deep  levels,  and  especially  the  so-called  juvenile 
waters  (i.e.  water  of  volcanic  origin)  must  have  the  power  of 
dissolving  quartz  to  some  extent.  Hot  springs  in  volcanic 
regions  (Iceland,  the  Yellowstone  Park,  and  New  Zealand)  often 
contain  much  dissolved  silica,  which  is  deposited  as  sinter 
around  the  outflow  of  the  spring,  and  this  may  be  in  part  derived 
from  direct  solution  of  quartz  at  great  depths,  though  much 
of  it  probably  comes  from  hydrolytic  decomposition  of  silicates, 
especially  felspars. 

The  felspar  group  includes  a  large  number  of  minerals  of 
varying  composition  and  varying  degrees  of  stability,  hence 
they  are  affected  by  weathering  in  various  ways.  From  the 
practical  point  of  view  the  most  important  of  these  changes  is 
the  formation  of  kaolinite  from  orthoclase  or  albite,  a  process 
which  results  in  the  production  of  great  beds  of  china-clay. 
The  composition  of  orthoclase  is  most  conveniently  repre- 
sented by  the  formula  K20  .  A1203  .  6Si02,  while  kaolinite  is 
a  hydrated  silicate  of  alumina  represented  by  the  formula 
A1203 .  2Si02 .  2H20. 

The  reaction  by  which  kaolinite  is  formed  has  been  very 


IT]  WEATHERING  53 

generally  attributed  to  the  influence  of  carbon  dioxide  dissolved 
in  water,  but  according  to  the  more  modern  view  it  is  to  be 
explained  as  due  to  water  alone ;  it  is  in  fact  to  be  regarded  as 
a  case  of  hydrolysis.  The  change  can  then  be  represented  by 
the  following  equation  : 

K20  .  A1203  .  6Si02  +  3H20 

-  2KOH  +  4Si02  +  A1203  .  2Si02 .  2H20, 

the  resultant  products  being  potassium  hydroxide,  silica  and 
kaolinite.  Now  silica  in  the  non-crystalline  form  is  readily 
soluble  in  caustic  potash,  or  to  state  the  case  more  correctly, 
potash  and  silica  combine  to  form  potassium  silicate,  which  is 
soluble  in  water,  and  is  removed,  leaving  only  kaolinite  in  the 
solid  form.  Sometimes  this  mineral  is  found  crystalline, 
while  in  other  cases  it  occurs  as  the  amorphous  modification, 
halloysite,  which  has  the  same  composition.  The  crystallization 
or  otherwise  of  this  mineral  is  probably  determined  by  the 
temperature  at  which  the  change  takes  place,  as  crystals  are 
more  common  in  cases  where  there  are  indications  of  the 
pneumatolytic  action  of  steam  at  a  high  temperature. 

This  hydrolytic  decomposition  takes  place  more  or  less 
whenever  water  is  in  contact  with  felspar,  but  in  most  cases 
with  extreme  slowness.  It  is  only  when  the  temperature  is 
high,  owing  to  volcanic,  solfataric  or  hydrothermal  action  near 
the  surface  or  to  pneumatolysis  at  greater  depths,  that  it  takes 
place  at  all  rapidly.  Under  favourable  conditions  however  it 
may  result  in  the  formation  of  great  masses  of  china-clay,  as 
in  Devon  and  Cornwall,  Saxony  and  near  Limoges. 

Another  and  perhaps  even  commoner  type  of  decomposition 
affecting  orthoclase  or  microcline  is  conversion  into  an  aggregate 
of  minute  flakes  of  white  mica.  This  change  involves  the 
removal  of  a  part  only  of  the  potash  together  with  two  thirds 
of  the  silica.  The  composition  of  typical  muscovite  is  approxi- 
mately 2H20  .  K20  .  3A1203  .  6Si02, 

3  [K20  .  A1203  .  6Si02]  +  2H20 

=  2K20  +  2H20  .  K20  .  3A1203  .  6Si02  +  12Si02. 
The  resulting  products  at  ordinary  temperatures  are  therefore 
muscovite  and  colloid  silica,  but  at  high  temperatures  the 


54  WEATHEKTNG  [CH. 

silica  may  crystallize  as  quartz,  forming  a  quartz-mica  rock,  or 
greisen.  The  former  case  is  the  more  common  and  this  only 
is  of  importance  in  normal  weathering  and  soil-formation. 
A  precisely  similar  reaction  occurs  in  the  case  of  soda-felspar, 
yielding  a  soda-mica,  paragonite,  which  in  minute  flakes  is 
indistinguishable  from  muscovite.  This  decomposition  of 
alkali-felspars  furnishes  much  of  the  finely-divided  micaceous 
substance  which  is  so  commonly  a  constituent  of  soils,  of 
modern  sediments  and  of  the  older  rocks  formed  from  similar 
sediments1. 

The  plagioclase  felspars  are  mixed  crystals  of  two  com- 
ponents, albite  and  anorthite,  in  varying  proportions.  The 
albite  molecule  is  Na20  .  A1203  .  6Si02,  while  the  anorthite 
molecule  is  CaO  .  A1203  .  2Si02.  These  two  molecules  are 
differently  affected  by  weathering  agents,  and  give  rise  to 
different  products.  This  subject  is  complex  and  not  well 
understood;  the  albite  molecule  appears  to  form  kaolinite  or 
mica,  according  to  circumstances,  as  in  the  case  of  orthoclase, 
while  the  lime-felspar  molecule  gives  rise  to  a  variety  of  products, 
such  as  epidote,  chlorite  and  even  calcite. 

Under  the  influence  of  certain  peculiar  types  of  weathering, 
whose  nature  is  even  yet  not  properly  understood,  the  felspars 
give  rise,  not  to  silicates,  as  previously  described,  but  to  various 
hydroxides  of  aluminium,  the  most  important  being  hydrargillite 
or  gibbsite,  A1203  .  3H20,  and  bauxite,  A1203  .  2H20.  This 
type  of  weathering  is  specially  characteristic  of  the  laterite 
deposits  of  tropical  regions2,  and  appears  therefore  in  all 
probability  to  be  determined  by  climatic  conditions,  or  possibly 
even  by  the  agency  of  bacteria3.  (For  a  discussion  of  the  origin 
of  laterite  see  pp.  57  and  115.) 

It  has  often  been  observed  that  the  decomposition  products 
of  lime-bearing  plagioclase  include  some  newly  formed  albite, 
this  molecule  having  apparently  recrystallized  as  such,  while 
the  lime-molecule  has  formed  compounds  belonging  to  other 

1  Hutchings,  Geol.  Mag.   1894,  pp.  36  and  64. 

2  Bauer,  "Beit-rage  zur  Geologie  der  Seychellen-Inseln,  und  besonders  zur 
Kenntniss  der  Laterite,"  Neues  Jahrb.  fiir  Min.  1898,  p.  168. 

3  Holland,  "Constitution  of  Laterite,"  Geol.  Mag.  1903,  p.  59. 


IT 


WEATHERING  55 


mineral  groups.  This  process  is  closely  analogous  to  the 
albitization  of  igneous  rocks  of  basic  composition1. 

The  decomposition  of  lime-bearing  plagioclase  felspar  is 
perhaps  the  most  important  of  all  sources  of  lime  compounds 
in  the  stratified  and  other  sedimentary  deposits.  To  this  we 
must  look  for  the  primal  origin  of  the  great  masses  of  calcium 
carbonate  which  constitute  the  calcareous  rocks,  limestone, 
dolomite,  calc-sinter  and  travertine,  as  well  as  the  thick  beds  of 
gypsum  and  anhydrite  and  the  lime-content  of  all  fresh  waters 
and  of  the  sea. 

The  mica  group.  It  is  remarkable  that  a  great  difference 
of  behaviour  towards  weathering  agents  is  found  in  the  members 
of  the  mica  group.  Stated  in  the  most  general  terms,  the 
colourless  or  very  pale  alkali-micas  of  the  muscovite  subgroup 
are  particularly  stable  minerals,  being  very  little  affected  by 
weathering  and  consequently  persisting  through  many  geological 
cycles;  their  final  disappearance  is  due  to  their  very  perfect 
cleavage,  which  causes  them  to  be  easily  abraded,  rather  than 
to  chemical  change.  Indeed  alkali-micas  are  a  very  common 
product  of  the  chemical  alteration  of  silicate  minerals,  fel- 
spars, nepheline,  cordierite,  garnet,  and  many  others.  On  the 
other  hand  the  iron  and  magnesian  micas  of  the  biotite  group 
are  very  unstable,  being  easily  decomposed  to  chlorite,  epidote 
and  other  hydrous  silicates.  Consequently  they  are  not  common 
constituents  of  sediments  or  of  soils.  The  easy  decomposition 
of  biotite  sets  free  iron  and  magnesia,  and  what  is  of  more 
importance  agriculturally,  a  considerable  amount  of  potash, 
which  exists  in  the  soil  in  a  form  readily  available  as  plant 
food,  probably  as  solutions  of  potassium  salts  absorbed  by  the 
argillaceous  or  organic  constituents  of  the  soil. 

The  amphibole  and  pyroxene  groups.  As  a  matter  of 
convenience  and  to  save  repetition  the  minerals  of  these  two 
groups  may  be  treated  together,  since  they  are  very  similar  in 
chemical  composition  and  in  mineralogical  characters,  yielding 
eventually  the  same  weathering  products.  Essentially  they  are 
all  mixed  silicates  of  magnesia,  iron  and  lime,  often  with  soda 

1  Bailey  and  Grabham,  "Albitization  of  the  Plagioclase  Felspars,"  Geol. 
Mag.  1909,  p.  250  (with  further  references). 


56  WEATHERING  [CH. 

and  alumina.  The  amphiboles  and  pyroxenes  are  polymorphous 
forms  of  similar  compounds  and  can  be  converted  one  into  the 
other  under  varying  conditions  of  temperature  and  pressure. 
Thus  a  very  common  change  in  igneous  rocks  is  the  conversion 
of  augite  into  hornblende  (uralitization).  By  weathering  both 
amphiboles  and  pyroxenes  are  converted  into  various  silicates, 
such  as  epidote,  chlorite  and  serpentine,  often  with  separation 
of  iron  in  the  form  of  oxides  or  hydroxides  (magnetite,  haematite, 
limonite),  together  with  carbonates  and  other  compounds  of 
very  uncertain  character  and  composition.  The  weathering  of 
the  amphiboles  and  pyroxenes  of  the  igneous  rocks  forms  a 
very  important  source  of  iron  and  magnesia  in  sediments  and 
in  soils. 

Olivine.  This  mineral  is  very  unstable  under  ordinary 
conditions  and  readily  undergoes  decomposition.  Olivine  is  of 
complex  composition,  being  best  described  as  a  mixture  of  the 
isomorphous  orthosilicates  of  magnesia,  iron  and  lime.  Each  of 
these  molecules  however  undergoes  a  different  kind  of  decom- 
position ;  stated  in  general  terms  the  magnesia  molecule  forms 
serpentine;  the  iron,  which  is  in  the  ferrous  state,  undergoes 
oxidation  and  hydration,  forming  various  oxides  and  hydroxides 
of  iron,  while  the  lime  molecule  commonly  gives  rise  to  carbonate 
(calcite).  Serpentinization  is  generally  attributed  to  the  action 
of  carbon  dioxide  dissolved  in  water,  the  reaction  being  repre- 
sented by  the  following  equation : 

2[2MgO  .  Si02]  +  C02  +  2H20  =  3MgO  .  2Si02 .  2H20  +  MgC03. 

Olivine  Serpentine 

The  change  involves  a  loss  of  magnesia,  since  the  carbonate 
is  fairly  soluble,  and  also  a  marked  alteration  of  volume  which 
has  a  shattering  effect  on  the  rock.  Cases  are  also  known  in 
which  original  crystals  of  olivine  have  been  replaced  by  dolomite, 
showing  that  under  certain  conditions  the  magnesium  silicate 
may  be  wholly  changed  to  carbonate. 

The  mineral  olivine  is  only  found  in  igneous  rocks  and  by 
its  decomposition  it  gives  rise  to  compounds  of  magnesia,  iron 
and  lime  of  some  importance  as  soil-formers. 


n]  WEATHERING  57 

The  weathering  of  silicates.  This  subject  has  been  investi- 
gated from  the  chemical  side  by  Van  Bemmelen1;  this  author 
treated  soils  of  different  types  by  successive  extractions  with 
acids  and  alkalies  of  increasing  strength,  chiefly  hydrochloric 
acid  and  caustic  soda.  He  then  determined  the  ratio  of  silica 
to  alumina  in  the  extracts,  in  the  endeavour  to  ascertain  whether 
the  existence  of  compounds  of  constant  composition  could  be 
demonstrated  in  the  weathered  soils.  The  results  were  some- 
what conflicting,  but,  after  examination  of  a  great  number  of 
samples,  it  was  concluded  that  three  distinct  types  of  weathering 
could  be  recognized,  as  follows : 

(1)  Ordinary  weathering,  in  which  the  ratio  of  silica  to 
alumina    is    approximately    3:1.     This    kind    of    weathering 
prevails  mostly  in  moist,  temperate  climates. 

(2)  Kaolinitic  weathering,  yielding  products  with  a  ratio 
of   2:1;    the  most  important  of  these  are  kaolinite  and  its 
amorphous  form  halloysite,  which  may  be  represented  by  the 
formula  A1203  .  2Si02 .  2H20.     The  determining  climatic  factor 
is  here  uncertain. 

(3)  Lateritic  weathering;    in  soils  of  this  type  the  chief 
constituents  are  hydroxides  and  hydrates  of  alumina,  such  as 
gibbsite    (hydrargillite)    and    bauxite.     Silica    when    present 
occurs  almost  exclusively  as  unweathered  minerals.     Lateritic 
weathering  is  specially  characteristic  of  tropical  regions  with 
distinct  wet  and  dry  seasons. 

No  conclusions  could  be  formed  as  to  the  state  of  aggregation 
of  the  bases  calcium,  magnesium,  iron,  potassium  and  sodium  in 
weathered  soils.  It  is  quite  uncertain  whether  they  exist  as 
crystalline  compounds  or  as  colloids,  or  merely  as  solutions 
adsorbed  by  the  aluminous  compounds  or  by  the  humus. 
Microscopic  investigation  of  the  constituent  particles  of  clays 
by  optical  and  other  methods  has  revealed  the  existence  of 
crystalline  minerals,  especially  kaolinite,  chlorite,  various 
mica-like  substances,  silicates  of  iron  and  of  lime,  magnetite, 

1  Van  Bemmelen,  "Beitrage  zur  Kenntniss  der  Verwitterungsprodukte  der 
Silikate  in  Ton-,  Vulkanischen  und  Lateritboden,"  Zeits.  jwr  anorg.  Chemie, 
vol.  XLH.  1904,  p.  265;  "Die  Verwitterung  der  Tonboden,"  ibid.  vol.  LXII.  1909, 
p.  221. 


58  WEATHERING  [CH. 

carbonates  of  lime  and  magnesia  and  others,  besides  minute 
unweathered  crystal-grains  of  most  of  the  common  rock-forming 
minerals,  especially  quartz,  felspar  and  mica.  The  identification 
of  the  mineral  constituents  of  weathered  soils  is  a  subject  on 
which  much  work  is  needed,  but  the  practical  difficulties  in  the 
way  are  great,  owing  to  the  minute  size  of  the  particles,  and  the 
presence  of  much  amorphous  material,  possibly  in  the  colloidal 
state,  such  as  the  kaolin- jelly  of  Ramann1. 

The  weathering  of  non-silicate  minerals.  Many  of  the 
minerals  which  are  stable  under  normal  conditions  of  tempera- 
ture and  pressure  are  not  silicates,  but  belong  to  other  classes 
of  chemical  compounds,  some  of  the  most  important  being 
oxides,  hydroxides,  carbonates,  sulphates  and  chlorides.  These 
minerals  naturally  undergo  changes  differing  much  in  their 
nature  from  those  affecting  the  silicates.  Since  these  minerals 
are  stable  under  the  given  conditions  the  tendency  is  towards 
their  formation  rather  than  their  destruction,  and  it  is  necessary 
to  treat  them  from  a  somewhat  different  standpoint. 

Magnetite  is  a  stable  mineral  occurring  as  an  original 
constituent  in  rocks  of  both  igneous  and  sedimentary  origin. 
Under  certain  circumstances  it  may  be  oxidized  to  haematite,  or 
it  may  become  hydrated  and  form  limonite  or  other  iron 
hydroxides,  but  commonly  it  is  passed  on  as  a  detrital  con- 
stituent from  one  rock  to  another  without  change. 

Ferrous  disulphide  in  the  form  of  pyrites  is  a  very  stable 
mineral  and  tends  to  be  produced  as  a  result  of  low  grades  of 
metamorphism,  but  when  existing  in  the  form  of  marcasite  it 
is  very  unstable,  readily  undergoing  oxidation  to  ferrous  sulphate 
and  ultimately  to  sulphuric  acid.  When  abundant  therefore 
it  can  play  an  important  part  in  soil  changes. 

The  weathering  of  the  carbonates  is  a  simple  process,  being 
for  the  most  part  solution  in  water,  generally  aided  by  the 
presence  of  carbon  dioxide,  as  explained  on  p.  47.  This  process 
really  comes  under  the  heading  of  erosion  rather  .than  of 
weathering  in  the  strict  sense.  Rock-salt  and  gypsum  are 
still  more  soluble  than  the  carbonates  and  the  same  remark 
applies  with  even  more  force. 

1  Ramann,  Bodenkunde,  3rd  edition,  Berlin,  1911,  p.  245. 


n]  WEATHEKING  59 

Minerals  unaffected  by  weathering.  Besides  the  minerals 
enumerated  in  the  list  previously  given,  many  of  the  common 
rocks  contain  a  greater  or  less  proportion  of  a  great  variety  of 
other  minerals,  many  of  which  are  not  affected  to  any  appreciable 
extent  by  weathering  agencies  or  by  chemical  changes  of  any 
kind.  Many  of  these  stable  species  originate  in  igneous  rocks,  for 
example  garnet,  zircon,  rutile  and  many  others.  Another  group 
is  formed  by  metamorphism  and  pneumatolysis  of  both  igneous 
and  sedimentary  rocks,  e.g.  tourmaline,  staurolite,  kyanite, 
spinel  and  again  garnet,  while  a  third  group  includes  minerals 
formed  in  mineral  veins,  e.g.  cassiterite,  and  a  host  of  others. 
Since  these  minerals  are  almost  indestructible  they  pass  with 
little  change  from  one  rock  to  another  during  a  cycle  of  de- 
nudation, deposition  and  cementation ;  hence  they  are  common 
constituents  of  sediments,  but  usually  only  in  small  quantities. 
From  the  agricultural  point  of  view  they  are  not  important, 
though  many  substances  of  great  economic  value,  such  as  gold, 
diamond  and  many  other  gems  and  tinstone,  come  under  this 
heading.  Owing  to  their  high  density  such  minerals  often 
undergo  a  kind  of  natural  concentration  in  gravels  and  sands, 
and  give  rise  to  important  industries.  Interesting  conclusions 
as  to  the  source  and  origin  of  sands  and  of  soils  can  sometimes 
be  drawn  from  a  study  of  these  mineral  constituents,  which  are 
easily  separated  by  special  methods1. 

1  Hatch  and  Rastall,  Textbook  of  Petrology,  vol.  n,  The  Sedimentary  Rocks. 
London,  1913,  appendix. 


CHAPTER   III 

TRANSPORT  AND  CORRASION 

The  process  of  denudation,  the  destruction  of  the  earth's 
surface,  consists  of  three  stages ;  the  first  of  these,  weathering,  is 
fully  described  in  the  last  chapter.  The  second  stage  is  transport 
and  the  third,  corrasion.  The  word  transport  explains  itself; 
it  is  the  removal  by  gravity,  wind,  water  or  ice  of  the  material 
loosened  by  weathering.  While. in  movement  this  material  is 
enabled  to  do  destructive  work  by  virtue  of  its  mechanical 
energy  and  this  work  is  summed  up  in  the  term  corrasion. 
The  geological  effects  of  transport  and  corrasion  are  so  closely 
interwoven  that  it  is  most  convenient  to  treat  them  together. 
Any  other  course  would  involve  much  needless  repetition. 

As  in  the  case  of  weathering,  transport  also  is  controlled 
largely  by  climate  and  altitude.  The  chief  agents  involved  in 
each  case  may  be  summarized  as  follows: 

Tropical  zone Running  water. 

Arid  zone          Wind. 

Temperate  zone  ...  Running  water. 

Arctic  zone       ...         ...  Ice. 

Gravity  is  of  course  equally  operative  in  all  areas,  and  all 
transport,  except  by  wind,  is  ultimately  due  to  gravity. 

As  before  pointed  out  the  relative  importance  of  weathering 
and  transport  in  any  given  area  is  largely  a  question  of  climate 
and  the  consequent  presence  or  absence  of  vegetation.  Trans- 
port is  at  a  minimum  in  the  tropics,  where  the  thick  vegetation 
is  protective  and  weathered  material  accumulates  to  a  great 
thickness.  On  the  other  hand  it  is  at  a  maximum  in  regions 
subjected  to  considerable  variations  of  climate,  where  all  the 


CH.  m]  TRANSPORT  AND   CORRASION  61 

agents  have  full  play  in  turn.  It  is  obvious  also  that  the  con- 
figuration of  the  land  must  have  a  great  influence  in  determining 
the  amount  of  work  done  by  water  and  by  ice,  since  the  energy 
of  both  of  these  agents  depends  on  their  velocity,  and  this  in 
its  turn  is  controlled  by  the  slope.  Gravity  and  water  can  only 
carry  material  to  a  lower  level ;  on  the  other  hand  wind,  and,  to 
a  less  extent,  ice,  can  transport  material  to  a  higher  level.  The 
general  tendency  however  is  nearly  always  downwards,  and  the 
final  stage  of  transport  is  deposition  in  the  sea. 

The  processes  of  corrasion  are  somewhat  more  difficult  to 
follow,  since  they  are  not  usually  conspicuous  when  in  actual 
operation.  In  general  however  it  is  fairly  obvious  that  a  river 
has  carved  out  its  own  bed,  and  the  nature  of  the  processes  here 
involved  may  now  be  considered.  The  simplest  of  all  is  solution 
and  in  certain  cases  this  is  undoubtedly  of  importance,  especially 
when  a  river  runs  over  limestone  rocks.  Most  corrasion  is 
however  purely  mechanical,  being  due  to  the  kinetic  energy  of 
the  water,  ice  or  wind  and  of  the  rock-fragments  transported 
by  these  agents.  Pure  water,  running  over  a  smooth  rock, 
without  material  in  suspension,  would  have  little  or  no  erosive 
effect,  owing  to  the  absence  of  friction,  but  when  it  is  carrying 
and  pushing  along  grains  of  sand,  pebbles  and  boulders,  these 
wear  away  the  rocks  over  which  they  travel,  acting  like  a  file 
or  sandpaper.  In  the  case  of  ice  this  effect  is  still  more  con- 
spicuous, as  shown  by  the  scratched  and  grooved  rock-surfaces 
seen  in  glaciated  regions.  The  erosive  effect  of  wind-blown 
sand  is  also  well  known.  Running  water  and  ice  also  exert 
what  may  be  described  as  a  plucking  action  on  the  rocks  over 
which  they  travel,  especially  if  these  are  well  jointed.  Frag- 
ments of  various  sizes,  bounded  by  joints,  are  thus  torn  away 
from  their  beds  and  carried  along  to  perform  work  in  their 
turn.  A  conspicuous  feature  of  water-borne  pebbles  is  their 
rounding  by  mutual  attrition,  and  consequent  reduction  in 
size.  Streams  also  enlarge  and  widen  their  beds  by  under- 
mining the  banks,  especially  when  flowing  through  soft  material. 

Besides  the  actual  cutting  away  of  the  beds  of  well-defined 
rivers  and  streams  there  is  constantly  in  operation  a  general 
and  slow  degradation  of  the  surface  of  the  land  due  to  gravity 


62  TRANSPORT   AND   CORRASION  [CH. 

and  rain.  The  effect  of  rain-wash  is  shown  by  the  muddy  state 
of  rivers  during  floods,  when  vast  quantities  of  finely  divided 
soil-material  are  carried  away  from  the  land  and  deposited  in 
the  sea.  The  lowering  of  the  general  surface  of  the  land  is  in 
the  main  due  to  the  washing  away  of  soil  by  rain-water. 

Besides  all  these  processes  belonging  to  the  land,  the  geologist 
must  also  take  into  account  transport  and  erosion  by  the  sea. 
This  is  a  subject  of  much  practical  interest,  especially  to  land- 
owners and  farmers  near  the  coast.  Even  within  historic 
times  coast  erosion  has  brought  about  great  geographical 
changes  in  the  British  Isles  and  elsewhere  (see  p.  76). 

Transport  and  soil-formation.  These  two  processes  are 
mutually  interdependent  and  it  is  impossible  to  overrate  the 
importance  of  a  correct  understanding  of  their  relationships. 
Sometimes  they  are  antagonistic,  in  other  instances  they  work 
together  for  a  common  end.  In  general  terms  it  may  be  said 
that  the  tendency  of  transport  is  to  take  the  soil  away  from  one 
place  and  to  deposit  it  somewhere  else.  It  is  evident  that  there 
must  be  limits  to  this,  but  these  limits  are  not  actually  reached 
until  the  material  has  been  carried  far  out  to  sea,  beyond  the 
influence  of  waves,  tides  or  currents.  Before  this  condition  is 
reached  any  material  which  has  been  deposited  is  always  liable 
to  disturbance  owing  to  varying  conditions. 

At  great  elevations  little  true  soil  is  formed,  and  in  hilly 
regions  there  is  always  a  tendency  for  the  soil  to  creep  downhill 
and  to  accumulate  on  the  floors  of  valleys  and  very  often  also 
in  lakes,  if  such  exist.  Alluvial  flats  formed  by  partial  or  com- 
plete filling  up  of  lakes  are  a  common  feature  of  hilly  districts 
and  they  are  composed  largely  of  transported  soil.  In  the 
middle  and  lower  parts  of  the  course  of  a  river,  where  the  grade 
is  less  steep,  much  deposition  of  soil-material  often  takes  place, 
forming  wide  spreads  of  alluvium,  while  deltas  and  deposits  in 
estuaries  also  consist  for  the  most  part  of  material  carried 
down  from  the  land  surface  of  the  upper  part  of  the  basin. 

From  these  considerations  it  is  evident  that  the  formation  of 
soil  in  any  given  area  depends  on  the  relation  between  weathering 
and  transport.  Where  weathering  is  excessive,  soil  will  tend 
to  accumulate  to  a  great  thickness,  while  the  other  extreme  is 


in]  TRANSPORT   AND   CORRASION  63 

exemplified  by  some  regions,  usually  high-lying,  where  transport 
is  so  much  in  excess  that  the  surface  consists  of  bare  rock. 
The  maintenance  of  a  constant  thickness  of  soil  must  be 
dependent  on  a  delicate  balance  between  the  geological  processes, 
and  probably  under  natural  conditions  the  thickness  of  the  soil 
layer  is  always  either  increasing  or  diminishing,  though  in  most 
instances  very  slowly.  It  is  quite  evident  however  that 
unusually  heavy  or  long-continued  rainfall  must  have  a  serious 
effect  in  removing  the  finely  divided  portion  of  the  soil,  in  all 
regions  except  perhaps  in  perfectly  level  plains.  The  sudden  and 
violent  downpours  commonly  known  as  "cloud  bursts,"  where 
a  great  volume  of  rain  is  concentrated  into  a  small  area  for  a 
short  space  of  time,  sometimes  produce  absolutely  disastrous 
effects,  removing  all  the  soil  and  loose  subsoil  down  to  the 
solid  rock,  and  rendering  the  land  useless  for  agricultural 
purposes.  Cloud  bursts  are  fortunately  rare  in  temperate 
climates,  but  in  some  arid  regions  they  are  not  uncommon. 

Denudation.  The  combined  effects  of  weathering,  transport 
and  corrasion  may  be  summed  up  in  the  general  term  denudation ; 
this  term  in  its  widest  sense  implies  the  general  degradation 
and  destruction  of  the  land  areas  of  the  world  and  the  transfer 
of  their  material  to  the  ocean  basins,  where  deposition  comes 
into  play  and  gives  rise  to  new  solid  masses,  namely,  sediments 
and  eventually  rocks.  The  effects  of  denudation  as  seen  on  the 
land  are  conveniently  expressed  by  the  term  earth-sculpture. 
This  includes  the  development  of  streams  and  rivers,  the  carving 
out  of  hills  and  valleys  and  in  short  the  formation  of  the  surface 
relief  of  the  land,  however  this  may  originate,  upheaval  and 
fracture  being  of  course  excluded. 

In  temperate  climates,  such  as  our  own,  rivers  are  by  far 
the  most  important  agents  of  denudation  and  land-sculpture, 
and  it  will  be  well  to  begin  with  a  consideration  of  their  origin 
and  development.  When  a  new  land- area  is  formed  by  uplift 
of  the  sea-floor  it  will  consist  of  smooth  sheets  of  sediment 
inclined  in  various  directions,  according  to  the  form  of  the 
uplifted  area.  The  rain  falling  on  the  land  will  tend  to  collect 
in  any  accidental  depressions  of  the  surface  and  to  run  down  the 
steepest  slope.  This  will  be  in  general  down  the  dip  of  the 


64 


TRANSPOKT  AND   COKRASION 


[CH. 


t 


f 


1 


I 


rocks,  and  such  primary  streams  may  be  called  dip- streams, 

or  consequent  streams,  since  they 
are  the  direct  consequence  of 
the  uplift.  Another  type  of 
consequent  stream  is  formed 
when  the  rocks  are  crumpled 
up  into  folds  with  inclined  axes, 
the  streams  then  running  along 
the  synclines.  The  lower  part 
of  the  Thames,  below  Reading, 
is  an  example  of  this.  Such 
longitudinal  valleys  are  very 
common  in  mountain  regions. 

As  time  goes  on  the  inequali- 
ties of  the  surface  increase  owing 
to  the  constant  removal  of 
weathered  material,  and  sub- 
sidiary or  tributary  valleys  de- 
velop, often  running  into  the 
main  valleys  more  or  less  at 
right  angles;  these  are  called 
subsequent  or  strike- streams  (since  they  are  at  right  angles  to  the 
dip  streams  they  must  be,  by  definition,  parallel  to  the  strike  of 
the  rocks).  These  again  eventually  come  to  possess  tributaries 
of  the  third  order,  and  so  on  indefinitely.  The  final  result  is  a 
network  of  streams,  such  as  may  be  seen  on  a  map  of  any 
well- watered  country,  running  generally  from  the  high  ground 
or  watershed  to  the  sea,  but  showing  much  variation  in  detail 
and  many  minor  deviations,  owing  to  differences  in  the  hardness 
of  the  rocks  over  which  they  run  and  to  other  causes. 

The  hardness  of  rocks  is  a  feature  of  fundamental  importance 
in  the  study  of  river  development,  and  may  be  stated  as  follows : 
hard  rocks  tend  to  stand  up  as  hills  and  soft  rocks  to  form 
valleys.  This  somewhat  obvious  fact  is  dignified  in  America 
with  the  title  of  the  Law  of  Structures.  Furthermore  when  a 
valley  is  carved  out  of  a  series  of  rocks  of  varying  degrees  of 
hardness,  the  part  of  the  valley  in  the  hard  rocks  is  narrow 
and  steep-sided,  often  forming  a  gorge  in  extreme  cases,  while 


Fig.  18.     Simple  dip-streams 
resulting  from  uplift. 


Ill] 


TRANSPORT  AND  CORRASION 


65 


in  the  softer  rocks,  the  valley  is  wide  and  open  with  gently 
sloping  sides. 


1 

, 

1 

1 

1  ~-^-__-^-^_ 

I    ~* 

\. 

r  — 

1 

1 

. 

1 
1  .  

• 

I 

1       

'       ~^~~ 

- 

1 

Fig.  19.     Development  of  strike-streams  as  tributaries  of  dip-streams.     The 
broken  line  is  the  primary  watershed. 


Fig.  20.     A  highly  developed  river  system,  showing  a  network  of  streams : 
the  broken  line  indicates  the  primary  watershed. 

During  the  earlier  stages  of  its  existence  a  river  gradually 
and  continuously  deepens  its  bed,  but  this  can  only  go  on  up 
R.A.  G.  5 


66  TRANSPORT  AND   CORRASION  [CH. 

to  a  certain  point.  The  character  of  the  bed,  when  considered 
in  vertical  section,  depends  on  the  age  of  the  river.  At  first 
the  soft  rocks  are  cut  away  more  quickly  than  the  hard  ones, 
hence  a  young  river  often  shows  an  alternation  of  steep  rocky 
gorges  and  wider  stretches  with  a  gentler  and  more  uniform 
slope.  Specially  hard  strata  often  form  rapids  and  waterfalls, 
and  the  existence  of  these  in  the  course  of  a  river  may  be  taken 
as  an  indication  of  youth. 


Fig.  21.     Profile  of  a  young  river  showing  inequalities  in  the  grade. 

Ultimately  all  inequalities  are  levelled  down  to  a  uniform 
slope  and  a  mature  river  which  can  cut  down  no  further  in  any 
part  of  its  course  is  said  to  have  reached  the  base-line  of  erosion. 


Fig.  22.     Profile  of  an  old  river  approaching  the  base-line  of  erosion. 

Probably  however  no  actual  rivers,  at  least  in  this  country, 
have  quite  reached  this  stage  throughout  their  whole  length; 
all  appear  to  be  capable  of  some  amount  of  downward  erosion 
in  the  upper  parts  of  their  courses,  and  those  originating  in 
hilly  and  still  more  in  mountainous  regions  are  still  actively 
engaged  in  eroding  their  channels,  as  shown  by  the  prevalence 
of  waterfalls  in  such  regions. 

When  a  river  has  reached  its  base  line  throughout  a  con- 
siderable part  of  its  course  it  still  possesses  energy,  which  must 
be  utilized  somehow.  Since  the  river  can  no  longer  cut  down- 
wards the  energy  is  employed  in  cutting  sideways,  that  is,  in 
lateral  corrasion.  The  bed  of  the  river  is  thus  widened  and 
also  tends  to  become  more  sinuous.  It  is  seldom  that  the 
course  of  a  river  is  quite  straight  and  any  accidental  projection 
of  the  bank  on  one  side  will  throw  the  current  over  towards  the 
opposite  bank,  producing  an  indentation  on  this  side  and  leaving 
slack  water  on  the  other  side.  In  this  slack  water  deposition 


Ill 


TRANSPORT  AND   CORRASION 


67 


goes  on,  forming  first  a  mud  bank  or  gravel  bed,  and  finally 
building  up  alluvial  land.  This  process  is  cumulative,  the  bends 
becoming  more  and  more  accen- 
tuated as  time  goes  on,  while  the 
whole  system  of  curves  also  tends 
to  travel  down  stream.  In  course 
of  time  a  river  may  thus  work 
over  a  very  considerable  area  on 
either  side  of  its  mean  channel, 
producing  the  large  alluvial  flats 
so  characteristic  of  the  lower  parts 
of  many  valleys.  These  exag- 
gerated bends  are  called  meanders 
and  are  often  extraordinarily 
acute;  at  times  the  neck  of  land 
between  two  reaches  of  the  river 
becomes  so  narrow  as  to  be  broken 
through  by  a  flood  and  the  channel 
is  temporarily  shortened  and 
straightened,  leaving  isolated 
curved  patches  of  water,  such  as 
are  called  "ox-bows"  along  the  course  of  the  Mississippi.  The 
form  of  a  valley  in  which  meandering  has  occurred  for  a  long 
time  is  quite  characteristic,  consisting  of  a  flat  alluvial  plain  in 


Fig.  23.  Diagram  to  show  develop- 
ment of  meanders  in  the  course 
of  a  river:  the  curves  gradually 
becoming  more  and  more  ac- 
centuated. 


Fig.  24.     Crescent-shaped  "ox-bow"  formed  by  shortening  and  straightening 
of  a  river  bed.     The  dotted  lines  show  the  course  before  the  diversion. 

which  a  winding  river  flows,  while  the  sides  of  the  valley  rise 
up  from  the  plain  as  bluffs  with  a  comparatively  steep  slope. 
In  this  way  have  originated  those  broad  strips  of  fertile  alluvial 

5-2 


68  TRANSPORT  AND   CORRASION  [CH. 

land  which  border  the  courses  of  many  rivers,  such  as  the 
Thames,  the  Trent  and  the  Yorkshire  Ouse.  They  consist  of 
river  silt  down  to  a  depth  usually  equal  to  the  maximum  depth 
of  the  channel  of  the  river  itself.  This  land  is  usually  somewhat 
swampy,  but  when  efficiently  drained  it  is  often  remarkably 
rich. 

***>_          «    ____  ***" 

%y:frta£?Sig^ 

*fy/////////$/SSfr/fW/f///f/ifftSf///Sff//f/fW/ft//fSS////f{l\  ' 


Fig.  25.  Cross  section  of  the  valley  of  an  old  river;  a  meander  plain  bounded 
by  bluffs.  The  dotted  portion  represents  the  alluvium  formed  by  the 
river,  which  is  seen  at  a,  but  has  at  different  times  occupied  every  possible 
position  within  the  limits  of  the  meander  belt. 

From  what  has  been  said  above  it  will  readily  appear  that 
the  relief  of  a  surface  produced  by  river  denudation  depends 
mainly  on  two  factors  ;  the  length  of  time  during  which  earth- 
sculpture  has  been  proceeding  and  the  original  structure  of  the 
area  in  relation  to  the  varying  hardness  and  character  of  the 
rocks  composing  it.  No  doubt  the  general  lowering  of  the  land 
is  an  extraordinarily  slow  process;  it  has  been  estimated  on 
what  appear  to  be  fairly  reliable  data  at  an  average  of  1  foot 
in  6000  years.  Such  a  rate  will  obviously  produce  no  visible 
effect  during  an  ordinary  lifetime,  and  it  is  difficult  or  impossible 
to  establish  the  existence  of  any  real  change,  even  from  the 
oldest  historical  records.  Geological  time  however  is  unlimited, 
and  in  theory  at  any  rate,  given  sufficient  time,  even  the  highest 
mountains  should  be  worn  away.  It  is  however  a  matter  of 
dispute  whether  land-denudation  alone,  without  the  assistance 
of  the  sea,  has  ever  been  able  to  reduce  a  hilly  country  to  a 
plain.  It  would  appear  probable  that  in  most  cases  some 
disturbing  factor,  such  as  a  general  uplift  or  subsidence,  is 
introduced  before  this  consummation  can  be  reached. 

It  is  impossible  in  the  space  here  available  to  give  a  full 
account  of  the  important  subject  of  river  development  and  of 
denudation  by  water-action.  Of  late  years  the  theory  of 
denudation  has  been  extensively  developed  in  America,  where 
the  geological  structure  is  simpler  and  the  conditions  somewhat 
more  favourable  than  in  Britain,  with  its  marked  local  variations 


m]  TRANSPORT   AND   CORRASION  69 

of  climate  and  great  complexity  of  geological  structure.  In 
the  British  Isles  also  ice-erosion  has  played  a  considerable  part 
in  modifying  the  relief  of  the  land.  At  the  present  time  the 
geological  effects  of  ice  and  water  have  not  been  satisfactorily 
discriminated  in  this  country,  and  in  many  cases  there  is  still 
much  dispute  as  to  the  precise  nature  of  the  principal  agent 
concerned  in  the  production  of  many  of  the  prominent  features 
of  our  valley  systems1. 

Water  denudation  and  topography.  In  regions  of  temperate 
climate,  where  the  effects  of  ice  are  excluded,  the  form  of  the 
land  surface  is  mainly  due  to  the  action  of  running  water. 
Owing  however  to  variations  of  original  rock-structure  the 
forms  produced  show  endless  diversity,  depending  on  relative 
hardness  of  strata,  inclination  and  folding  of  the  rocks,  faults, 
landslips  and  many  other  factors.  It  may  be  said  in  general 
terms  that  the  broad  outlines  of  the  distribution  of  the  land 
and  the  arrangement  of  the  principal  mountains  and  watersheds 
depend  primarily  on  earth  movement,  while  the  details  of 
coastal  forms  are  mainly  due  to  the  sea,  but  the  minor  features 
of  the  land,  the  hills,  valleys,  canons,  gorges  and  waterfalls  are 
determined  by  streams  and  rain. 

In  a  region  where  denudation  has  been  going  on  for  a  long 
time  the  valleys  are  wide  and  open  and  the  outlines  of  the  hills 
rounded,  smooth  and  flowing.  Serrated  ridges  and  narrow 
rocky  valleys  indicate  a  youthful  drainage  system,  where  time 
has  not  yet  sufficed  to  wear  down  the  asperities  of  the  surface. 
Waterfalls  in  the  course  of  a  river  are  almost  always  indications 
of  youth ;  in  old  rivers  and  in  those  at  and  near  base-level  the 
course  is  worn  down  to  one  uniform  curve.  Coming  to  the 
details  of  rock-erosion  in  actual  river-beds,  special  importance 
attaches  to  pot-holes.  These  are  circular  hollows  produced  in 
the  rock  by  the  gyrations  of  stones  in  eddies;  they  are  often 
very  deep  and  undoubtedly  play  an  important  part  in  the 
scooping  out  oj:  many  river-beds,  where  the  current  is  rapid  and 
pebbles  abundant. 

1  For  a  concise  account  of  the  geological  effects  of  running  water  see  Bonney, 
The  Work  of  Rain  and  Rivers.,  Cambridge  Manuals  of  Science  and  Literature 
Cambridge,  1912. 


70  TRANSPORT  AND   CORRASION  [CH. 

The  term  canon  is  one  which  has  been  much  abused.  In 
America  it  is  now  used  to  describe  almost  any  kind  of  valley, 
but  in  this  country  it  generally  connotes  the  idea  of  a  deep 
and  narrow  chasm  in  the  earth ;  such  would  be  perhaps  better 
designated  a  gorge.  Such  deep,  narrow  valleys  are  formed  in 
two  principal  ways :  firstly,  if  the  rocks  through  which  the  river 
runs  are  very  hard  and  resistant,  the  denudation  of  the 
surrounding  slopes  is  small  and  the  sides  of  the  valley  remain 
steep ;  secondly,  if  a  river  running  from  a  normally  wet  region 
cuts  through  a  dry  area,  denudation  of  the  surrounding  country 
is  again  at  a  minimum  and  the  same  result  follows.  Some  of 
the  greatest  gorges  of  the  world,  such  as  the  Colorado  Canon, 
are  due  to  a  combination  of  special  causes.  In  the  first  place  the 
Colorado  river  flows  from  a  mountain  region  into  a  dry,  almost 
desert  tract,  but  furthermore,  after  the  river  had  established 
its  course  and  reached  base-level,  an  uplift  of  the  whole  country 
took  place,  so  that  the  river  was  enabled  again  to  cut  deeply 
into  the  strata.  The  gorge  is  now  some  300  miles  long  and  has 
a  maximum  depth  of  about  6000  feet. 

In  some  cases  rivers  flow  straight  towards  mountain  ranges 
and  cut  through  them  in  narrow  deep  gorges.  In  such  an  instance 
the  only  possible  explanation  seems  to  be  that  the  river  is  actually 
older  than  the  mountains,  and  was  strong  enough  to  keep  open 
its  original  course,  as  the  mountain  chain  rose  under  the  influence 
of  crumpling  of  the  crust.  Such  is  the  common  explanation  of 
the  Iron  Gate  of  the  Danube  and  the  valleys  of  the  Indus  and 
the  Bramaputra,  both  of  which  rise  on  the  north  of  the  Himalaya 
and  cut  right  through  the  range. 

It  would  be  easy  to  go  on  multiplying  indefinitely  instances 
of  peculiar  topography  directly  due  to  river  denudation  under 
special  conditions,  but  space  will  not  allow  of  any  further 
amplification  of  the  subject.  Enough  has  been  said  to  bring 
out  the  main  points  at  issue.  For  a  detailed  treatment  reference 
must  be  made  to  some  general  text-book  of  geology. 

Denudation  in  limestone  regions.  Owing  to  the  ready 
solubility  of  calcium  carbonate  in  water,  the  results  of  water- 
erosion  of  limestones  present  certain  peculiar  characteristics. 
Besides  being  soluble,  limestones  are  usually  very  well  jointed. 


in]  TRANSPORT   AND   CORRASION  71 

The  joints  are  produced  in  the  first  place  by  shrinkage,  but 
they  are  rapidly  widened,  often  to  considerable  depths,  by 
solution,  and  consequently  any  rain  falling  on,  or  streams 
running  over,  the  surface  soon  disappear  down  these  open 
joints;  in  limestone  countries  most  of  the  water  circulation 
is  underground  and  the  surface  remains  dry  and  waterless, 
often  consisting  merely  of  bare  rock.  Thus  is  produced  that 
topographical  type,  perhaps  most  highly  developed  in  the 
region  known  as  the  Karst,  to  the  east  of  the  Adriatic,  hence 
known  as  the  Karst  type  of  scenery.  It  is  also  very  well  seen 
in  many  parts  of  the  British  Isles  where  the  Carboniferous 
Limestone  forms  the  surface  rocks,  as  in  west  Yorkshire,  north 
Lancashire,  Westmorland,  Derbyshire,  the  Mendip  Hills  and 
elsewhere.  Highly  characteristic  are  the  grikes  and  dints  of 
west  Yorkshire,  Lancashire  and  Westmorland,  elevated  plateaux 
of  limestone,  consisting  of  bare  rock,  with  many  wide  open 
joints,  in  which  alone  plants  find  sufficient  moisture  for  their 
existence.  A  very  fine  example  of  this  is  seen  on  Ingleborough, 
and  this  region  is  also  remarkable  for  its  caves  and  swallow-holes, 
both  features  highly  characteristic  of  limestone  regions.  The 
formation  of  caves  depends  on  the  widening  of  joints  and 
fissures  by  solution,  aided  by  the  collapse  of  undermined  portions 
of  rock.  Very  commonly  an  underground  river  runs  through 
a  cave,  often  falling  into  it  down  a  swallow-hole,  as  at  Gaping 
Ghyll  on  Ingleborough.  These  swallow-holes  are  vertical 
shafts,  sometimes  two  or  three  hundred  feet  deep.  They 
originate  as  vertical  joints,  and  are  enlarged  and  deepened  by 
stream-erosion.  When  a  limestone  plateau  is  covered  by  a 
thin  layer  of  drift  or  peat,  swallow-holes  may  be  formed  in  the 
limestone  beneath  and  the  superficial  layer  sinks  into  them, 
forming  conical  hollows ;  these,  when  numerous,  give  the  surface 
a  curious  pitted  appearance.  Similar  structures  are  sometimes 
formed  in  the  Chalk.  . 

The  topography  of  Chalk  areas  is  of  a  somewhat  special 
type  and,  owing  to  its  wide  occurrence  in  England,  worthy  of 
some  description.  Chalk  is  a  limestone  and  therefore  soluble 
in  water.  Its  drainage  is  consequently  for  the  most  part 
subterranean,  as  in  other  limestone  areas.  But  it  is  likewise 


72  TRANSPORT  AND   CORRASION  [CH. 

soft  and  therefore,  except  in  sea-cliffs,  it  does  not  form  rocky 
scarps  like  the  Carboniferous  and  other  hard  limestones.  Chalk 
hills,  like  the  Downs  and  the  Chilterns,  are  consequently  smooth 
and  undulating  in  outline,  though  often  fairly  steep.  Surface 
streams  are  generally  absent;  nevertheless  a  well-developed 
valley  system  is  generally  to  be  seen.  This  is  somewhat 
difficult  to  account  for,  but  is  generally  explained  on  the 
supposition  that  the  valleys  were  formed  when  the  ground  was 
frozen  to  a  considerable  depth,  during  the  glacial  period.  Then 
the  streams  were  compelled  to  run  on  the  surface,  being  unable 
to  sink  into  the  frozen  rock,  and  thus  carved  out  the  valleys 
in  the  usual  way.  A  similar  explanation  will  account  for  the 
dry  limestone  valleys  of  the  north-west  of  England.  The 
origin  and  characters  of  the  soils  found  lying  on  the  Chalk  are 
discussed  elsewhere  (see  Chapter  xiv). 

Denudation  in  arid  regions.  In  those  parts  of  the  globe 
where  the  rainfall  is  deficient  the  geological  conditions  are  quite 
special  and  of  a  peculiar  character.  This  type  of  denudation 
is  not  found  in  the  British  Isles,  and  is  only  seen  in  its  full 
development  within  the  desert  belt.  Roughly  speaking  an 
arid  region  may  be  defined  as  one  where  the  average  annual 
rainfall  is  less  than  10  inches.  Such  conditions  prevail  for 
example  over  a  large  part  of  both  north  and  south  Africa  and 
in  Australia.  The  day  temperature  is  very  high  and,  owing  to 
the  clear  skies,  there  is  a  rapid  fall  after  sunset.  Hence  changes 
of  temperature  are  very  sudden  and  produce  important  effects 
by  alternate  expansion  and  contraction  of  the  rocks,  causing 
conspicuous  shattering.  This  is  the  principal  weathering  agent, 
and  there  is  comparatively  little  chemical  action,  owing  to  the 
absence  of  water  and  vegetation.  The  fragmental  material  is 
therefore  very  fresh  and  undecomposed.  The  chief  agent  of 
transport  is  wind,  though  gravity  plays  an  important  part  in 
mountainous  regions.  Under  such  conditions  sand  is  formed 
in  enormous  quantities  and  transported  for  long  distances  by 
wind.  While  in  movement  the  sand  performs  a  certain  amount 
of  work  in  erosion,  but  according  to  the  best  authorities  the  actual 
amount  of  denudation  due  to  sand  is  generally  insignificant, 
though  its  superficial  effects  are  striking,  consisting  of  rounded 


m]  TRANSPORT  AND   CORRASION  73 

and  polished  surfaces.  In  regions  where  desert,  topography  is 
fully  developed,  agriculture  is  impossible  and  the  subject  need 
not  concern  us  further.  There  are  however  many  cultivated 
regions,  sometimes  comparatively  fertile,,  where  the  rainfall 
comes  below  the  limit  specified,  e.g.  parts  of  the  Union  of  South 
Africa  and  of  Australia.  Here  for  part  of  the  year  the  climate 
is  very  dry  and  for  many  months  no  rain  may  fall.  The 
greater  part  of  the  rainfall  is  concentrated  into  a  short  season, 
and  the  downpours  when  they  do  occur  are  often  violent. 
Hence  the  effects  of  denudation  are  somewhat  complicated. 
In  the  dry  season  denudation  of  the  arid  type  is  dominant, 
while  in  the  wet  season  water  action  is  conspicuous  and  floods 
often  produce  very  well-marked  effects.  In  such  regions 
geological  processes  are  on  the  whole  probably  more  rapid  than 
in  temperate  climates  and  the  topography  is  highly  accentuated, 
as  in  the  mountain  regions  of  Cape  Colony. 

Ice  as  an  agent  of  denudation.  Here  we  touch  upon  one  of 
the  most  difficult  and  controversial  of  all  geological  subjects. 
Of  the  potency  of  ice  as  an  agent  of  transport  there  is  no  question ; 
it  is  obvious  on  the  most  cursory  view  of  a  glacier  or  of  an  iceberg, 
but  when  we  come  to  the  consideration  of  erosion  by  ice,  it  is 
another  matter.  This  is  a  subject  on  which  the  most  contra- 
dictory views  are  held  by  authorities  of  the  highest  rank. 

In  the  high  mountain  regions  where  glaciers  are  found,  and 
in  the  arctic  lands,  the  chief  agent  of  rock-disintegration  is 
frost;  fragments  fall  from  the  mountain  slopes  on  to  the  ice, 
and  are  carried  along  by  it,  either  on  the  surface  or  within  the 
body  of  the  ice.  These  form  the  accumulations  known  as 
moraines.  As  the  ice  moves  slowly  downwards  the  stones  are 
carried  along  with  it,  to  be  deposited  eventually  at  the  point 
where  the  ice  melts,  forming  the  terminal  moraine.  This  is 
usually  a  crescent-shaped  mound,  running  across  the  valley  in 
which  the  glacier  terminates,  and  consisting  of  more  or  less 
angular  fragments  derived  from  the  surface  moraines  of  the 
ice ;  but  the  terminal  moraine  itself  includes  a  good  deal  of  finely 
divided  material,  and  the  streams  running  from  the  end  of  the 
ice  are  always  turbid  with  fine  mud,  the  so-called  rock-flour. 
The  question  next  arises  what  the  source  of  this  may  be. 


74  TRANSPORT  AND   CORRASION  [CH. 

Part  of  it  is  certainly  formed  by  the  mutual  attrition  of  frag- 
ments, but  this  cannot  account  for  much  and  we  are  driven  to 
conclude  that  the  glacier  really  does  erode  its  bed  as  a  whole, 
mainly  by  means  of  stones  embedded  in  its  lower  surface  and 
dragged  along,  though  also  doubtless  to  a  considerable  extent 
by  plucking,  i.e.  by  tearing  away  of  blocks  along  joint  planes. 
No  one  doubts  at  any  rate  that  glaciers  have  an  important 
and  conspicuous  effect  in  modifying  the  surface  over  which  they 
move.  This  is  shown  by  the  rounding,  polishing  and  grooving 
of  projecting  rocks,  in  any  region  where  glaciers  have  recently 
existed;  the  main  controversy  is  as  to  whether  the  erosive 
effect  of  ice  is  purely  superficial,  merely  putting  a  final  polish 
on  to  features  already  formed  by  water  action,  or  whether  ice- 
erosion  has  been  the  principal  agent  of  land-sculpture  in  regions 
where  ice  has  once  existed.  The  question  cannot  be  answered 
yet,  since  the  evidence  is  too  conflicting,  and  it  is  not  of  much 
interest  to  the  agriculturist,  who  is  concerned  more  with  the 
effects  of  ice  as  an  agent  of  deposition. 


Fig.  26.  On  the  left  a  V-shaped  valley  due  to  water-erosion :  on  the  right 
a  valley  with  a  U-shaped  section  in  the  lower  part,  due  to  deepening  by 
ice  of  a  river  valley.  The  dotted  line  shows  the  form  of  the  original  river 
valley. 

It  is  generally  believed  at  the  present  time  that  the  form  of 
the  cross-section  of  a  valley  gives  some  indication  of  the  nature 
of  the  agent  by  which  it  was  formed.  A  valley  with  uniformly 
sloping  sides  and  very  narrow  at  the  bottom,  such  as  is  best 
described  by  the  term  V-shaped  valley,  is  supposed  to  be  due 
entirely  to  water;  such  valleys  are  often  winding  as  well  as 
steep  and  narrow  and  projecting  spurs  are  conspicuous.  On  the 
other  hand  a  U-shaped  valley  is  supposed  to  be  due  principally 
to  ice ;  here  the  sides  are  very  steep,  often  indeed  precipitous, 
but  there  is  a  fairly  flat  floor,  often  of  quite  considerable  width. 


in]  TRANSPORT  AND   CORRASION  75 

U-shaped  valleys  are  generally  straight  and  the  tributary 
streams  fall  into  them  abruptly,  often  as  waterfalls;  projecting 
spurs  do  not  exist,  having  been  planed  away  by  the  ice. 

Evidences  of  the  former  existence  of  glaciers  are  conspicuous 
in  most  of  the  mountainous  regions  of  our  own  country,  especially 
in  Wales,  the  Lake  District  and  Scotland.  They  consist  of  the 
characteristic  polished  and  scratched  surfaces  before  mentioned, 
together  with  roches  moutonnees  and  perched  blocks.  A  roche 


Fig.  27.     A  roche  moutonnee,  with  a  perched   block  resting  on   it.     The  ice 
moved  from  right  to  left. 

moutonnee  is  a  projecting  knob  of  rock  that  has  been  smoothed 
and  polished  on  the  side  from  which  the  ice  came,  while  the 
sheltered  side  is  left  in  a  rough  and  jagged  condition;  perched 
blocks  are  masses  of  rock  left  lying  by  the  ice  on  the  surface 
of  the  ground,  often  after  having  been  transported  for  long 
distances.  The  general  term  boulder  is  often  employed  to 
connote  masses  of  rock  of  considerable  size  that  have  been 
transported  from  a  distance  and  either  left  lying  on  the  surface, 
or  embedded  in  gravel,  sand  or  clay.  Besides  the  foregoing 
there  are  often  to  be  seen  moraines  of  all  kinds,  whose  origin  is 
indicated  by  their  characteristic  form,  and  also  many  accumu- 
lations of  gravel,  sand  and  other  material,  either  deposited 
directly  by  the  glaciers  or  by  the  streams  running  from  them. 
Even  the  lowland  tracts  of  Great  Britain  north  of  the  Thames 
are  for  the  most  part  covered  by  a  sheet  of  glacial  deposits, 
largely  consisting  of  boulder  clay.  These  superficial  deposits 
are  of  enormous  agricultural  importance  and  their  characters 
are  fully  described  elsewhere  (see  Chapters  v  and  xvi).  They 
are  only  mentioned  here  as  physical  evidence  for  the  former 
existence  in  this  country  of  glacial  conditions. 


76  TRANSPORT  AND   CORRASION  [CH. 

Marine  denudation  and  coast-lines.  This  is  a  subject  of 
much  practical  significance  in  certain  maritime  regions,  although 
to  the  inland  agriculturist  it  is  not  a  matter  of  concern.  It  has 
long  been  known  that  great  changes  have  occurred  along  the 
coasts  of  the  British  Isles  and  of  late  years  the  subject  has  been 
considered  so  important  that  a  Royal  Commission  was  appointed 
to  carry  out  a  thorough  investigation.  The  final  report  of  this 
commission  shows  that  while  there  has  been  much  loss  of  land 
by  marine  erosion  in  certain  districts,  it  is  more  than  counter- 
balanced by  gain  in  other  localities,  so  that  the  total  area  of 
the  British  Isles  has  slightly  increased  in  recent  years.  How- 
ever it  is  poor  consolation  for  a  landowner  in  Yorkshire  or 
Norfolk  who  has  lost  a  whole  farm,  to  know  that  some  one  in 
South  Wales  or  Lancashire  has  gained  a  similar  or  larger  area. 
In  general  it  may  be  said  that  destruction  is  most  rapid  on  the 
projecting  parts  of  our  eastern  coasts,  in  Holderness,  Norfolk 
and  Suffolk,  while  the  gain  is  taking  place  chiefly  in  the  long 
estuaries  and  narrow  inlets  of  the  west  coast  of  England  and 
Wales1.  It  is  estimated  that  during  the  last  35  years  the  loss 
has  amounted  to  about  6600  acres,  while  the  area  gained  is 
estimated  at  48,000  acres.  Much  of  this  new  land  however  is 
as  yet  of  little  or  no  agricultural  value,  being  still  largely  mud 
flats  and  salt  marshes,  which  in  course  of  time  will  doubtless 
become  good,  fertile  land. 

Marine  denudation  is  due  to  three  principal  causes ;  waves, 
tides  and  currents;  these  usually  act  in  conjunction  and  their 
effects  are  often  separated  with  difficulty.  The  efficiency  of 
these  causes  naturally  depends  primarily  on  the  character  of 
the  rocks  on  which  they  act.  It  may  seem  a  platitude  to  say 
that  soft  rocks  are  eroded  more  quickly  than  hard  ones,  but 
nevertheless  this  simple  statement  is  the  main  foundation  of 
the  whole  subject.  Where  hard  and  soft  rocks  alternate  the 
coast-line  is  likely  to  be  indented,  especially  if  exposed  to  the 
full  fury  of  the  waves,  as  in  the  south-west  of  Ireland.  On  the 
other  hand  rocks  of  uniform  hardness  tend  to  give  rise  to  smooth, 
flowing  shore  lines,  as  on  the  east  of  England,  especially  in 

1  Final  Report  of  the  Royal  Commission  on  Coast  Erosion,  London,  1911, 
p.  158. 


in]  TRANSPORT  AND   CORRASION  77 

Northumberland,  Lincolnshire  and  Norfolk.  The  subject  is 
however  often  complicated  by  submergence  or  emergence  of 
land.  The  former  tends  to  form  an  indented  coast,  since  the 
sea  flows  up  river-valleys,  as  in  the  west  of  Scotland,  while 
emergence  lays  bare  a  smooth  area  of  sea-floor,  forming  a  flat 
coastal  plain  of  simple  outline,  as  in  the  eastern  United  States. 
Deposition  also  tends  to  fill  up  bays  and  estuaries  and  to  reduce 
inequalities  of  outline.  Thus  the  Wash,  once  a  deep  gulf 
running  up  nearly  as  far  as  Cambridge,  was  subsequently  for 
the  most  part  filled  up  by  deposition  of  sediment.  The  origin  of 
the  Fenland  is  discussed  elsewhere  (see  p.  128). 

The  most  serious  coast- erosion  now  taking  place  in  the 
British  Isles  is  seen  on  the  Yorkshire  coast  between  Flamborough 
Head  and  Spurn  Point,  and  on  the  coasts  of  Norfolk  and  Suffolk, 
from  Cromer  southwards.  In  both  these  areas  the  cliffs  consist 
of  soft  glacial  deposits,  boulder  clay,  sands  and  gravels,  which 
are  easily  washed  away  by  the  strong  southward  set  of  the  tidal 
currents.  It  is  estimated  in  the  report  of  the  Royal  Commission 
before  quoted  that  the  loss  of  land  in  Holderness  in  a  period  of 
45  years  amounts  to  about  770  acres,  and  in  East  Anglia  in  a 
little  over  30  years  nearly  1000  acres  have  been  swept  away1. 
In  both  areas  the  sites  of  well-known  mediaeval  towns  and 
villages  are  now  far  out  to  sea. 

Coast-erosion  is  mainly  due  to  two  causes;  actual  friction 
on  the  rocks  of  the  shore,  caused  by  the  transport  of  stones 
and  shingle  by  the  waves,  and  landslips  due  to  undermining 
at  the  base  of  the  cliff.  In  hard  and  well- jointed  rocks, 
especially  those  with  open  joints,  great  importance  is  to  be 
assigned  to  the  sudden  compression  of  air  within  the  cavities 
of  the  rock,  when  waves  dash  up  against  them.  This  cause  is 
specially  operative  when  cliffs  descend  into  deep  water,  with 
no  beach  at  the  base.  In  such  cases  lines  of  caves  are  often 
formed  somewhere  about  high  water  mark,  extending  far  inland. 
This  also  leads  to  undermining  and  falls  of  cliff.  Caves  are 
specially  common  in  limestone,  and  especially  in  Chalk  cliffs, 


1  Op.  cit.  p.  43.      The  figures  for  East  Anglia  are  generalized  from  the  data 
given  for  separate  counties,  with  slightly  varying  dates  in  each  case. 


78 


TRANSPORT   AND   CORRASION 


CH. 


as  at  Flamborough  Head  and  on  the  Antrim  coast  and  in  the 
Carboniferous  Limestone  in  South  Wales. 

While  wave-action  is  mostly  confined  to  the  lower  part  of 
the  cliff,  ordinary  denudation  by  rain  and  frost  is  acting  on  the 
upper  part,  and  wearing  it  away.  Hence  the  steepness  of  the 
cliff  depends  on  the  ratio  between  these  two  kinds  of  denudation. 


n 


Fig.  28.  On  the  left,  a  cliff  in  well-jointed  rock  of  uniform  texture;  on  the 
right  an  overhanging  cliff  formed  of  hard  rock  above  with  a  soft  layer 
below. 


Fig.  29. 


A  cliff  of  well- jointed  rock  with  a  platform  at  the  base  due  to  wave- 
action,  forming  a  beach. 


If  the  cliff  is  composed  of  one  kind  of  rock  from  top  to  bottom, 
a  hard  rock  will  form  a  steep  cliff,  owing  to  the  small  effect  of 
subaerial  denudation,  while  a  soft  rock  will  be  weathered  away 


in]  TRANSPORT  AND   CORRASION  79 

more  quickly  above,  forming  a  gently  sloping  cliff;  in  fact  in 
many  places  where  the  rocks  are  all  soft  there  is  no  cliff  ai  all, 
but  the  surface  of  the  land  slopes  down  evenly  and  continuously 
to  the  beach.  Some  of  the  steepest  of  all  cliffs  are  formed  where 
a  hard  rock  overlies  a  soft  one ;  these  cliffs  may  even  overhang, 
since  the  base  wears  away  more  quickly  than  the  upper  part. 
For  our  present  purpose  marine  deposition  is  of  far  greater 
importance  than  denudation,  so  that  all  consideration  of  the 
nature  of  sea-beaches  may  be  postponed  for  the  present.  The 
nature  of  coast  deposits  of  all  kinds  is  discussed  fully  in 
Chapter  iv. 


CHAPTER   IV 

SEDIMENTS 

The  sedimentary  deposits  in  general.  It  has  already  been 
explained  that  the  modem  sediments  and  the  ancient  rocks  of 
similar  character  have  been  formed  either  directly  or  indirectly 
from  the  materials  of  pre-existing  rocks,  and  that  the  primary 
source  of  all  this  material  is  to  be  sought  in  the  original  primitive 
crust  of  the  earth,  this  being  of  necessity  of  igneous  origin,  or 
at  any  rate  formed  at  a  high  temperature.  Consequently  the 
sediments  are  partly  composed  of  the  same  minerals  as  the 
igneous  rocks,  in  an  unaltered  state,  and  partly  of  the  products 
of  the  weathering  and  decomposition  of  these  minerals. 

Sediments  may  be  formed  in  a  large  number  of  different 
ways  and  the  resulting  products  show  much  variation  in 
character  and  composition.  As  a  matter  of  practical  con- 
venience they  are  generally  treated  under  three  headings 
according  to  their  manner  of  formation,  namely,  mechanical, 
chemical  and  organic. 

The  mechanical  sediments  include  those  deposits  formed  as 
a  result  of  denudation  and  deposition  by  running  water,  ice, 
wind  and  gravity;  the  chemical  sediments  are  formed  by 
evaporation  of  solutions,  precipitation  and  other  similar  pro- 
cesses of  purely  physical  and  chemical  nature,  while  the  organic 
sediments  are  directly  due  to  the  vital  activity  of  animals  and 
plants. 

An  alternative  method  of  classification,  and  one  perhaps 
more  strictly  scientific,  is  framed  according  to  the  conditions 
under  which  the  deposit  is  formed,  namely,  marine,  estuarine, 
fresh-water  and  terrestrial.  This  classification,  however,  if 


CH.  iv]  SEDIMENTS  81 

applied  in  detail,  involves  much  repetition,  since  very  similar 
deposits  are  formed  under  different  conditions. 

The  mechanical  sediments.  The  subdivisions  of  the 
mechanical  sediments,  according  to  the  classification  here 
adopted,  are  founded  on  the  size  of  the  component  fragments. 
Any  division  of  this  kind  must  of  course  be  purely  arbitrary 
and  all  possible  gradations  may  exist.  As  a  basis  for  the  groups 
are  taken  the  popular  and  generally  understood  terms,  boulders, 
pebbles  (or  gravel),  sand  and  mud.  The  limits  of  size  are 
approximately  as  follows: 

Boulders  are  more  than  4  inches  (10  centimetres)  in  diameter, 
i.e.  larger  than  the  human  fist. 

Pebbles  vary  from  4  inches  to  the  upper  limit  for  sand, 
about  one-tenth  of  an  inch. 

Sand  includes  grains  smaller  than  this  and  yet  distinctly 
visible  to  the  unaided  eye. 

Mud  (or  dust  when  dry)  is  composed  of  microscopic  and 
ultramicroscopic  particles. 

The  above  is  the  nomenclature  applied  to  the  modern 
deposits  of  this  group,  and  the  older  consolidated  sedimentary 
rocks  have  been  formed  from  similar  deposits  by  various 
processes  of  hardening  and  cementation,  brought  about  by 
pressure  and  chemical  agents.  The  finest  types  of  sediment, 
dust  and  mud,  may  apparently  be  consolidated  by  pressure 
alone,  induced  by  the  weight  of  the  overlying  strata,  but  the 
coarser  sediments  can  only  be  welded  into  solid  rocks  by  actual 
deposition  of  material  in  their  interstices.  This  material  is  for 
the  most  part  deposited  from  solution  in  percolating  water, 
being  largely  derived  from  the  zone  of  weathering  near  the 
surface  and  carried  down  by  the  underground  circulation,  but 
also  in  some  cases  brought  up  from  great  depths  by  heated 
waters,  perhaps  of  volcanic  origin. 

The  chief  substances  which  thus  act  as  cements  are  silica, 
calcium  carbonate  and  various  oxides  and  hydroxides  of  iron, 
other  less  common  cements  being  gypsum  and  barium  sulphate. 
Sometimes  the  cements  are  deposited  in  the  amorphous  form, 
as  in  the  case  of  many  of  the  iron  compounds  and  opaline  silica, 
but  more  commonly  they  are  crystalline,  e.g.  quartz,  calcite, 

R.  A.G.  6 


82  SEDIMENTS  [CH. 

and  gypsum.  Sometimes  finely  divided  interstitial  muddy 
matter  acts  as  a  cement  for  the  larger  elements  of  the  rock,  but 
most  commonly  there  has  been  some  infiltration  as  well. 

The  coarser  types  of  mechanical  sediment,  when  cemented, 
form  conglomerate  if  the  boulders  or  pebbles  are  rounded,  and 
breccia  if  the  fragments  are  angular.  Hence  conglomerates 
are  formed  from  rounded  and  water-worn  boulders,  pebble 
deposits  and  gravel,  such  as  those  of  sea-beaches  and  of  rivers, 
whereas  breccias  are  chiefly  composed  of  scree-material  and 
similar  accumulations  due  to  frost  action  or  to  dry  weathering 
in  arid  regions.  The  pebbles  and  fragments  composing  them 
may  consist  of  any  kind  of  rock  and  no  distinctive  names  are 
in  use  for  varieties  differing  in  this  respect.  The  cement  also 
may  be  of  any  kind. 

The  forms  assumed  by  boulders  and  pebbles  show  some 
variety  according  to  the  conditions  of  formation.  Those  due  to 
water  action  are  for  the  most  part  well  rounded,  sometimes 
almost  spherical,  but  more  commonly  ovoid  or  ellipsoidal. 
This  shape  is  due  to  a  combination  of  rolling  and  sliding  move- 
ments. The  fragments  composing  glacial  deposits  are  generally 
angular  and  often  scratched,  while  rock  fragments  that  have 
been  acted  on  by  wind-blown  sand  are  polished  and  faceted, 
and  sometimes  worn  into  strange  shapes.  In  residual  deposits 
due  to  weathering  of  rocks  in  place,  the  fragments  may  have 
any  shape  whatever,  but  they  are  commonly  more  or  less 
rounded.  The  most  sharply  angular  fragments  are  found  in 
screes,  which  are  due  to  weathering  by  frost  action  or  changes 
of  temperature,  followed  by  accumulation  of  the  material  at 
the  foot  of  steep  slopes  in  mountain  regions.  In  considering 
the  forms  of  pebbles  in  relation  to  conditions  of  formation  it 
must  always  be  remembered  that  the  pebbles  may  have  been 
derived  already  rounded  from  some  older  deposit,  so  that  they 
are  more  rounded  than  they  ought  to  be.  Many  pebbles  of 
specially  resistant  rocks  have  certainly  been  handed  on  from 
one  deposit  to  another  through  several  geological  cycles. 

Conglomerates  and  breccias.  As  before  stated  these  are 
formed  by  cementation  of  boulder  and  pebble  deposits  of  ancient 
date.  These  are  abundant  among  many  of  the  rock  formations 


iv]  SEDIMENTS  83 

of  all  ages  composing  the  geological  series.  A  few  representative 
examples  may  be  described  briefly.  Since  pebble  deposits  are 
usually  formed  either  in  shallow  water,  salt  or  fresh,  or  actually 
on  land,  they  are  very  commonly  associated  with  unconformities 
and  breaks  in  the  stratigraphical  succession.  During  both 
submergence  and  emergence  of  land,  pebble  beaches  are  formed, 
and  at  the  base  of  a  series  of  marine  strata  there  is  usually  a 
conglomerate  resting  on  the  denuded  surface  of  the  older  rocks, 
while  the  sediment  becomes  finer  upwards.  Such  is  the  case, 
for  example,  where  the  marine  Cambrian  strata  rest  on  the 
pre- Cambrian  volcanic  rocks  in  parts  of  Wales  and  western 
England.  Similar  conglomerates  are  also  seen  at  the  base  of 
the  Carboniferous  in  west  Yorkshire,  and  in  many  other  instances 
too  numerous  to  mention.  Figs.  16  and  17  show  two  different 
ways  in  which  the  basal  conglomerate  may  be  related  to  the 
rocks  above  and  below.  Among  the  older  rock-systems  are 
also  found  conglomerates  that  appear  to  be  of  terrestrial  origin, 
having  been  formed  by  rivers.  Well-known  examples  are  the 
conglomerates  in  the  Old  Red  Sandstone  of  the  north  of  England 
and  of  Scotland,  and  the  Bunter  pebble  beds  in  the  Lower  Trias 
of  Devonshire,  Staffordshire  and  Cheshire;  these  are  often 
very  imperfectly  cemented,  easily  undergoing  disintegration 
to  form  loose  gravels  again.  The  "Hertfordshire  Pudding 
Stone"  of  Tertiary  age  is  a  very  remarkable  conglomerate, 
consisting  of  rounded  pebbles  of  flint  in  a  matrix  of  quartz  sand 
cemented  by  quartz.  One  of  the  best  known  examples  of  a 
breccia  is  the  "  Brockram  "  of  Permian  age,  found  in  Cumberland 
and  Westmorland;  it  consists  of  angular  fragments  of 
Carboniferous  Limestone,  more  or  less  converted  into  dolomite 
and  embedded  in  a  red  marly  substance  with  crystals  of  gypsum. 
It  was  probably  formed  in  a  salt  lake  at  the  foot  of  a  steep 
slope,  under  desert  conditions. 

Sands  and  sandstones.  The  rocks  formed  by  cementation 
of  sands  show  a  good  deal  of  variation  in  character ;  the  most 
typical  member  of  the  group  is  naturally  sandstone ;  the  meaning 
of  other  terms  employed  here  will  be  explained  shortly.  The 
most  common  cementing  materials  in  the  sandstones  and  other 
rocks  of  this  group  are  silica,  oxides  of  iron  and  calcium 

6—2 


84  SEDIMENTS  [CH. 

carbonate;  gypsum  and  barytes  as  cementing  materials  are  so 
rare  as  to  be  negligible  from  the  agricultural  point  of  view. 
The  cements  of  sandstones  are  almost  always  deposited  from 
solution  in  percolating  water,  and  the  hardness  of  the  resulting 
rock  naturally  depends  on  the  nature  of  the  cement  and  the 
extent  t6  which  the  interstices  between  the  grains  are  filled  up. 
The  softest  and  least  consolidated  sandstones  often  possess  a 
cement  of  limonite  or  some  other  iron  oxide,  deposited  only 
at  the  points  where  the  grains  are  actually  in  contact,  leaving 
considerable  interstitial  spaces;  frequently  in  sands  formed 
under  arid  conditions  each  grain  is  covered  by  a  skin  of  red 
ferric  oxide,  and  this  helps  to  bind  the  sands  into  a  more  or 
less  coherent 'rock.  In  many  soft  estuarine  and  marine  sand- 
stones the  cement  is  some  form  of  brown  hydrated  iron  oxide, 
e.g.  limonite,  and  this  occasionally  becomes  quite  hard,  giving 
rise  to  ferruginous  grits.  Sandstones  with  a  calcite  cement 
are  usually  fairly  hard,  since  the  calcite  is  commonly  deposited 
in  the  form  of  large  crystals,  each  enclosing  many  sand  grains, 
and  showing  a  characteristic  lustre-mottling  when  broken; 
such  rocks  are  often  called  calcareous  grits.  The  hard  sandstones 
of  the  older  rock-systems  generally  have  a  cement  of  silica  in 
the  form  of  quartz,  or  sometimes  a  mixture  of  quartz  and  finely 
divided  mica,  the  latter  being  formed  by  decomposition  of 
felspar.  A  hard  rock  of  compact  texture  composed  entirely  of 
grains  of  quartz  with  a  cement  of  the  same  mineral  is  generally 
called  a  quartzite,  though  some  writers  restrict  this  term  to  a 
rock  which  has  been  recrystallized  by  heat  or  pressure  (i.e.  a 
metamorphosed  sandstone). 

The  word  grit  is  very  often  used  as  a  descriptive  term  and 
a  good  deal  of  difference  of  opinion  exists  as  to  its  proper 
application.  Generally  it  appears  to  mean  a  sandstone  of 
somewhat  coarse  and  uneven  texture  (e.g.  the  Millstone  Grit 
of  the  Carboniferous  system)  characterized  by  a  rough,  gritty 
feeling  to  the  touch;  this  might  otherwise  be  designated  a 
pebbly  sandstone.  An  arkose  is  a  sandstone  or  grit  with 
abundant  fragments  of  felspar,  while  greywacke  is  a  somewhat 
old-fashioned  term  applied  to  the  grey  or  greenish  sandstones 
and  grits  so  common  in  the  oldest  sedimentary  formations. 


iv]  SEDIMENTS  85 

In  many  of  these  the  prevailing  green  colour  is  due  to  the 
presence  of  much  finely-divided  chlorite  in  the  cement,  probably 
derived  from  alteration  of  mica  and  other  ferromagnesian 
minerals. 

The  colour  and  general  appearance  of  a  cemented  sandstone 
depends  very  largely  on  the  character  of  the  cementing  material ; 
generally  speaking  quartzose  and  calcareous  sandstones  are 
white,  pale  grey  or  yellowish,  while  ferruginous  sandstones  are 
yellow,  brown  or  red. 

Sandstones,  especially  those  formed  in  a  shallow  sea,  often 
contain  abundant  grains  of  glauconite,  a  mineral  of  somewhat 
variable  and  indefinite  composition,  being  mainly  a  silicate  of 
potash  and  ferrous  iron.  The  presence  of  much  glauconite 
gives  a  distinctive  green  tinge  to  the  rock.  It  is  perhaps  best 
known  in  the  "Greensands"  of  the  Cretaceous  (see  Chapter  xiv), 
but  glauconite  is  found  in  marine  sands  of  almost  all  ages. 
Sands  rich  in  glauconite  are  often  associated  with  important 
phosphate  deposits,  as  will  be  explained  later,  and  the  presence 
of  this  mineral  is  a  useful  indicator  for  the  possible  presence  of 
phosphate  beds.  Even  if  phosphate  is  not  found  in  appreciable 
quantity  it  is  improbable  that  a  soil  derived  from  a  rock  rich 
in  glauconite  will  be  seriously  deficient  in  phosphoric  acid, 
unless  this  constituent  has  been  exhausted  by  crops. 

Many  sandstones  are  rich  in  flakes  of  mica,  and  a  large 
proportion  of  this  mineral  has  considerable  influence  in  deter- 
mining the  suitability  of  the  stone  for  various  economic  purposes. 
Flakes  of  mica  tend  to  accumulate  in  layers,  along  the  bedding 
planes  and  highly  micaceous  sandstones  show  a  strong  tendency 
to  split  into  flat  slabs,  which  are  commonly  known  as  flags. 
Many  other  well-bedded  sandstones,  without  much  mica,  are 
often  called  by  the  same  name,  and  some  of  the  flags  of  com- 
merce are  coarse  gritty  slates  or  even  finely  crystalline  mica- 
schists,  both  the  latter  groups  being  properly  metamorphic 
rocks.  The  best  flags  for  paving  purposes  are  found  in  the 
Coal  Measures  of  west  Yorkshire  and  in  the  Old  Red  Sandstone 
of  Caithness,  but  they  are  now  largely  superseded  by  various 
artificial  stones,  made  of  granite  chips  and  Portland  cement, 
cast  in  moulds  to  the  required  shape  and  thickness. 


86  SEDIMENTS  [CH. 

As  before  stated,  in  nearly  all  sands  and  sandstones  quartz 
is  by  far  the  most  abundant  mineral ;  next  come  felspar  and 
white  mica.  Among  the  less  abundant  minerals  various  com- 
pounds of  iron,  such  as  magnetite,  ilmenite  and  limonite  are 
perhaps  the  most  important.  Most  sands  also  contain  a 
small  proportion  of  other  hard  and  heavy  minerals,  such  as 
garnet,  tourmaline,  hornblende,  augite,  kyanite,  staurolite, 
rutile,  zircon  and  many  others.  These  are  for  the  most  part  of 
scientific  interest  only ;  a  few  of  them,  such  as  apatite,  may  by 
their  decomposition  yield  plant  food  in  soils,  but  most  are  very 
resistant  to  weathering  agents.  Many  recent  sea-sands  are 
very  rich  in  shell  fragments,  which  may  ultimately  yield  lime ; 
they  are  often  dissolved  out  before  cementation  begins.  Sands 
and  sandstones  of  ancient  date  are  as  a  rule  not  rich  in  fossils. 

Concerning  the  chemical  composition  of  the  sandy  deposits, 
there  is  little  to  be  said.  Since  quartz  is  almost  always  the 
chief  constituent  the  proportion  of  silica  is  necessarily  very 
high,  and  some  fresh  uncemented  sands,  siliceous  sandstones 
and  quartzites,  are  almost  pure  silica.  The  presence  of  felspars 
and  micas  introduces  alkalies  and  alumina,  while  the  other 
minerals  mentioned  above  contain  magnesia,  iron,  lime  and 
other  constituents.  But  the  proportion  of  all  these  is  low  in 
the  purer  sandstones,  rising  in  amount  in  the  impure  varieties, 
such  as  arkose  and  greywacke.  The  arkoses  in  particular 
sometimes  approximate  in  composition  to  igneous  rocks,  and 
by  their  decomposition  may  be  valuable  sources  of  potash, 
phosphoric  acid  and  lime.  However  the  plant  food  in  sandy 
rocks  and  in  sandy  soils  is  generally  in  a  form  not  readily 
available,  and  pure  sands,  whatever  their  actual  chemical 
composition  may  be,  are  commonly  almost  barren.  The  reason 
is  mainly  physical,  being  closely  connected  with  the  compara- 
tively large  size  of  the  particles  and  the  consequent  low  water- 
holding  capacity. 

The  nature  of  the  cementing  material  must  of  necessity  have 
a  strong  influence  on  the  bulk  composition  of  the  rock.  If  this 
is  ferruginous  or  calcareous  the  analysis  will  show  a  high 
proportion  of  iron  and  lime  respectively,  with  a  corresponding 
effect  on  the  nature  of  the  soil. 


iv]  SEDIMENTS  87 

Muds,  clays  and  shales.  These,  which  are  known  collectively 
as  the  argillaceous  deposits,  represent  various  degrees  of  altera- 
tion .of  the  finer  grained  deposits,  laid  down  in  the  sea  in  fairly 
deep  water,  at  some  distance  from  land,  or,  under  special 
conditions,  in  shallow  water  or  even  on  land.  In  general  terms 
however  it  may  be  said  that  the  argillaceous  rocks  are  generally 
marine.  They  are  formed  of  the  finest  material  derived  from 
denudation  of  the  land,  and  owing  to  the  minute  size  of  the 
particles,  the  exact  mineral  composition  is  often  a  matter  of 
uncertainty.  When  minerals  are  recognizable  under  high 
magnification  they  are  commonly  the  same  as  in  the  sands,  but 
there  is  generally  present,  in  addition,  more  or  less  of  a  structure- 
less jelly-like  substance,  the  so-called  "colloidal  clay."  As  to 
the  nature  and  even  the  real  existence  of  this  colloidal  clay 
there  has  been  much  dispute  (see  p.  57). 

The  marine  muds  are  generally  grey  or  bluish  in  colour ;  red 
and  green  muds  are  also  known  to  exist  in  the  sea,  but  they  are 
very  local  and  limited  in  their  distribution.  The  prevailing 
grey  tint  is  commonly  due  to  finely  disseminated  iron  sulphide, 
with  sometimes  carbonaceous  matter  in  addition,  derived  from 
the  decay  of  animals  and  plants.  When  partly  consolidated  by 
pressure  and  drying,  and  subsequently  uplifted  to  form  land, 
the  blue  and  grey  muds  give  rise  to  the  grey  clays  which  are  so 
common  in  many  districts  in  the  midlands  and  in  the  south  of 
England.  A  further  degree  of  hardening  converts  them  into  mud- 
stones,  or  if  laminated,  into  shale  (for  a  discussion  of  the  origin  of 
lamination  see  p.  16).  When  subjected  to  cleavage  these  rocks 
form  slates ;  the  latter  are  strictly  speaking  metamorphic  rocks. 

The  red  clays  and  so-called  "marls"  of  the  Trias  and  other 
formations  were  produced  in  a  different  way.  '  They  consisted 
originally  of  fine  dust  transported  by  wind  during  the  prevalence 
of  arid  conditions  and  either  piled  up  in  hollows  on  the  land 
surface  or  sometimes  deposited  in  salt  lakes.  This  accounts 
for  their  frequent  association  with  beds  of  rock-salt  and  gypsum, 
as  in  Cheshire  and  Worcestershire.  The  red  colour  is  due  to 
the  absence  of  any  organic  matter  to  reduce  the  iron  from  the 
ferric  to  the  ferrous  state,  and  may  be  taken  as  an  indication 
of  deposition  under  desert  conditions. 


88  SEDIMENTS  [CH. 

Muds  are  very  commonly  formed  on  a  large  scale  in  the 
estuaries  of  rivers,  owing  to  the  coagulating  effect  of  the  salt 
water  on  the  finely-divided  muddy  material  brought  down  by 
the  river,  leading  to  rapid  precipitation.  Estuarine  muds  give 
rise  to  clays  and  shales  much  like  those  of  marine  origin,  but 
distinguishable  by  the  nature  of  the  included  fossils,  which 
comprise  a  mixture  of  marine  and  fresh-water  or  even  terrestrial 
forms. 

The  deposits  in  large  fresh- water  lakes  generally  include 
beds  of  mud  and  clay;  these  often  contain  a  considerably 
higher  proportion  of  lime  than  the  muds  of  the  sea,  and  many 
of  them  are  more  properly  designated  marls,  in  the  true  sense 
of  this  term  (see  p.  93).  These  when  consolidated  form 
calcareous  mudstones  or  very  impure  limestones. 

Terrestrial  accumulations  belonging  to  this  class  are  mostly 
formed  by  wind,  a  notable  example  being  the  Loess  of  central 
Europe  and  Asia.  This  is  a  fine  brown  loam  without  strati- 
fication and  often  attaining  a  thickness  of  hundreds  or  even 
thousands  of  feet,  as  in  China.  It  is  believed  to  consist  of  dust 
of  glacial  origin,  redistributed  by  wind  during  warm  interglacial 
episodes.  When  modified  by  addition  of  organic  matter  it 
forms  the  famous  "black  earth"  of  Russia  (see  p.  148). 

The  chemical  composition  of  the  argillaceous  rocks  shows  a 
wide  range  of  variation,  but  nearly  all  varieties  are  distinguished 
by  a  fairly  large  proportion  of  alumina,  this  being  the  most 
characteristic  constituent,  while  silica  generally  averages  about 
50  per  cent.  Owing  to  the  presence  of  much  finely-divided 
felspar  and  mica,  the  clays  are  commonly  rather  rich  in  alkalies 
in  a  fairly  available  state,  and  clay  soils  are  rarely  deficient  in 
potash,  though  lime  is  often  in  small  amount.  A  very  important 
property  of  clays  is  their  power  of  taking  up  salts  from  solutions 
that  come  in  contact  with  them.  This  property,  which  is  known 
as  adsorption,  is  no  doubt  of  great  significance  in  the  study  of 
the  fertility  of  clays  and  loams.  It  seems  to  be  a  purely 
physical  process  and  not  true  chemical  combination.  The 
composition  of  clays  is  much  affected  by  the  manner  in  which 
they  have  been  formed  and  the  amount  of  chemical  change 
undergone  by  the  mineral  constituents  during  the  preliminary 


iv]  SEDIMENTS  89 

weathering.  The  most  highly  altered  are  the  so-called  residual 
clays.  These  are  the  insoluble  residues  of  highly  weathered 
rocks  and  are  often  of  very  peculiar  composition,  some  being 
even  derived  .  from  limestones.  The  calcium  carbonate  has 
been  leached  out  by  water  and  the  clay  is  nothing  but  the 
muddy  material  that  was  originally  enclosed  in  the  limestone 
at  the  time  of  its  formation. 

The  other  extreme  is  shown  by  the  clays  of  glacial  origin. 
These  are  formed  almost  entirely  by  mechanical  attrition,  the 
material  having  undergone  little  or  no  chemical  change.  Con- 
sequently there  has  been  no  leaching  and  all  the  original 
constituents  of  the  rock  may  be  present  in  a  comminuted  form. 
Such  clays  will  obviously  vary  in  composition  according  to  the 
character  of  the  parent  rock,  which  may  be  of  any  kind. 

Ordinary  marine  and  estuarine  clays  are  intermediate  in 
character;  some  of  the  material  consists  of  fresh  mineral 
chips  of  minute  size,  loosened  by  erosion,  while  another  part  is 
a  decomposed  residue  of  weathered  minerals.  In  many  ancient 
clays  and  shales  and  in  all  slates  there  has  been  also  a  develop- 
ment of  new  minerals,  largely  of  a  micaceous  nature,  formed  by 
chemical  reactions  between  the  original  constituents. 

Some  special  varieties  of  clay  are  worthy  of  brief  mention : 

China-clay  is  a  product  of  the  weathering  of  the  felspars  in 
granite;  its  formation  has  already  been  described  (see  p.  52). 

Pipe-clay  is  very  similar  in  appearance  to  china-clay,  being 
pure  white;  it  is  probably  also  very  similar  in  origin. 

Fire-clay  is  a  variety  with  an  unusually  low  proportion  of 
alkalies.  Bricks  made  of  this  clay  consequently  resist  heat  well 
and  are  used  for  lining  furnaces.  Beds  of  fire-clay  often  occur 
immediately  underneath  coal  seams.  These  are  supposed  to  be 
in  many  cases  the  soil  in  which  the  coal-forming  plants  grew, 
the  absence  of  alkalies  being  due  to  the  growth  of  the  plants. 

Fullers'  earth  is  a  peculiar  clay  which  does  not  become 
plastic  but  crumbles  away  when  wetted.  It  is  often  rather 
rich  in  magnesia,  lime  and  soda.  It  possesses  the  property  of 
removing  grease  from  cloth,  and  is  also  used  for  the  purpose 
of  decolourizing  dark  oils.  It  is  found  in  the  Jurassic  and 
Cretaceous  systems. 


90  SEDIMENTS  [CH. 

The  limestones.  This  group  includes  an  enormous  number 
of  rock-types,  very  variable  in  character  and  origin,  but  all 
agreeing  in  containing  a  large  proportion  of  calcium  carbonate. 
Limestones  may  be  of  mechanical,  chemical  or  organic  origin, 
and  may  be  formed  under  almost  any  set  of  physical  conditions. 
The  greater  part  are  marine,  but  they  may  be  formed  in  fresh 
water  or  even  on  dry  land,  though  this  is  not  common.  Owing 
to  the  solubility  of  calcium  carbonate  in  water  the  denudation 
of  limestone  regions  is  of  a  special  character  and  limestones 
sometimes  give  rise  to  very  peculiar  soils,  or  even  on  occasion 
to  no  soil  at  all,  the  surface  being  bare  rock. 

The  mechanically-formed  limestones  are  sediments  in  the 
true  sense  of  the  word.  They  consist  of  the  debris  of  older 
limestones,  loosened  by  weathering  and  transported  to  an  area 
of  deposition.  Some  of  them,  such  as  the  limestones  of  the 
Lias  in  Dorset,  are  consolidated  calcareous  muds,  derived 
probably  from  the  Carboniferous  and  older  limestones,  carried 
down  by  rivers  into  the  sea  and  there  deposited,  to  be  sub- 
sequently consolidated  by  pressure  and  drying.  The  Cornstones 
of  the  Old  Red  Sandstone  system  of  Hereford  and  South  Wales 
are  best  described  as  calcareous  breccias  on  a  small  scale, 
formed  in  a  very  similar  way.  It  seems  probable  from  the 
results  of  recent  research  that  limestones  formed  in  this  way  are 
more  common  than  was  formerly  believed.  It  is  often  difficult 
to  establish  any  clear  distinction  between  limestones  of  this  class 
and  those  formed  by  broken  fragments  of  calcareous  organisms, 
as  will  shortly  appear. 

The  chemically  formed  limestones  originate  chiefly  by 
precipitation  from  saturated  solutions.  When  calcium  car- 
bonate is  acted  on  by  water  containing  carbon  dioxide  a 
bicarbonate  is  formed,  thus  : 

CaC03  +  H20  +  C02  =  CaH2(C03)2 

and  this  salt  is  more  soluble  in  water  than  the  normal  carbonate. 
Since  the  amount  of  carbon  dioxide  that  water  can  hold  in 
solution  is  increased  by  pressure,  underground  waters  are  often 
rich  in  the  bicarbonate.  When  the  water  comes  to  the  surface 
as  a  spring  the  salt  decomposes,  the  gas  escaping  into  the  air; 


iv]  SEDIMENTS  91 

the  normal  carbonate  is  again  formed  and  is  precipitated  in 
the  solid  form.  Thus  originate  the  great  deposits  of  travertine 
and  calc-sinter  which  are  so  noteworthy  a  feature  of  some 
limestone  regions.  The  stalactites  and  stalagmites  of  caves 
originate  in  a  similar  way. 

Another  interesting  example  of  chemically  formed  calcareous 
deposits  is  afforded  by  the  "surface  limestones"  of  South  Africa 
and  other  hot,  dry  countries.  Owing  to  surface  evaporation 
water  rises  by  capillarity  from  below,  bringing  with  it  calcium 
carbonate  in  solution.  This  is  deposited  in  the  interstices  of 
the  soil  and  rocks  immediately  below  the  surface,  resulting  in 
the  formation  of  a  layer  of  calcareous  matter,  which  is  often 
very  hard,  and  is  an  effectual  bar  to  cultivation1. 

Many  limestones  of  various  ages,  and  especially  those  of  the 
Jurassic  system  (see  Chap,  xiu)  are  characterized  by  possessing 
what  is  known  as  oolitic  structure.  Such  rocks  show  a  strong 
resemblance  to  the  roe  of  a  fish,  whence  the  name.  Oolitic 
limestones  can  apparently  be  formed  in  several  ways.  Some 
of  them,  and  especially  the  coarser  types,  are  formed  by  the 
cementation  of  grains  derived  from  an  older  limestone,  and 
rounded  by  rolling  in  water;  they  are  in  fact  sands  composed 
of  grains  of  limestone  instead  of  quartz.  Others,  and  perhaps 
the  majority,  are  formed  by  a  chemical  process ;  water  becomes 
saturated  with  calcium  carbonate,  which  is  deposited  in  layers 
round  a  nucleus  of  any  foreign  substance,  such  as  a  quartz- 
grain  or  bit  of  shell.  These  show  under  the  microscope  a  well- 
developed  concentric  structure.  A  third  type  appears  to  be 
formed  by  the  activity  of  organisms,  probably  some  kind  of 
algae,  though  this  is  not  very  well  understood.  When  the 
grains  are  of  some  considerable  size  the  rock  is  often  called 
pisolite  (pea-stone).  Some  well-known  examples  of  pisolite  are 
formed  by  hot  springs,  as  at  Karlsbad  in  Bohemia  and  Vichy 
in  France. 

By  far  the  greater  part  of  the  limestones  are  formed  either 
directly  or  indirectly  through  the  agency  of  animals  and  plants. 
A  very  large  proportion  of  the  marine  invertebrata  possess  a 
calcareous  shell  or  skeleton  of  some  kind  and  after  the  death 

1  Rogers  and  Du  Toit,  Geology  of  Cape  Colony,  1909,  pp.  401-404. 


92  SEDIMENTS  [CH. 

of  the  animal  these  structures  accumulate  in  great  masses  on 
the  floor  of  the  sea,  forming  shell-beds  and  other  such  deposits, 
in  water  of  varying  but  not  very  great  depth.  Most  of  these 
are  loose  and  incoherent  at  first,  but  are  afterwards  cemented 
into  solid  rock  by  partial  solution  and  redeposition  of  the  calcium 
carbonate.  Some  calcareous  deposits  however  are  solid  and 
coherent  from  the  beginning,  the  best  example  of  this  class 
being  coral-rock.  Of  late  years  it  has  been  shown  that 
calcareous  algae  play  a  considerable  part  in  the  building  up  of 
coral-reefs. 

The  floors  of  many  shallow  seas,  such  as  the  English  Channel, 
are  largely  covered  by  layers  of  more  or  less  broken  shells, 
constituting  shell-sand,  which  is  a  true  mechanical  deposit,  but 
of  organic  origin.  From  this  there  is  a  regular  transition  to 
the  shell  banks  that  are  so  common  a  feature  of  shore  lines 
and  shallow  water,  especially  where  exposed  to  prevailing  winds, 
as  on  the  western  coasts  of  the  British  Isles  and  of  Holland. 
When  partially  solidified  these  give  rise  to  such  deposits  as  the 
Pliocene  Crags  of  Norfolk  and  Suffolk,  and  when  completely 
cemented  and  hardened  they  form  shell-limestones,  as  in  the 
Lower  and  Middle  Jurassic  (Lias,  Great  Oolite  and  other 
formations).  Sometimes  beds  of  oysters  and  other  similar 
bivalves  may  be  buried  in  sediment  and  preserved  as  shelly 
bands  among  clays,  shales  and  other  non-calcareous  deposits. 
Many  limestones  of  all  ages  have  been  formed  from  accumu- 
lations of  whole  and  broken  calcareous  organisms  in  water  of 
various  depths,  and  among  the  older  systems  these  animals  often 
belong  to  groups  now  rare  or  even  extinct.  A  notable  example 
is  afforded  by  the  crinoid  limestones  of  the  Lower  Carboniferous. 
Though  one  of  the  most  abundant  of  all  calcareous  animals  at 
that  time,  the  crinoids  are  now  rare  and  quite  negligible  as 
rock-builders.  Associated  with  them  are  great  numbers  of 
corals  and  brachiopods,  and  the  whole  mass  was  probably 
formed  in  clear  though  not  deep  water.  The  origin  of  the 
Wenlock  Limestone  has  given  rise  to  some  discussion ;  though 
rich  in  corals,  it  does  not  appear  to  have  been  a  true  coral-reef, 
but  was  perhaps  more  of  the  nature  of  a  shell-bank,  formed  in 
a  warm  or  tropical  sea. 


ivl  SEDIMENTS  93 

Chalk  is  a  very  special  variety  of  limestone,  of  a  white  or 
greyish  colour  and  of  peculiar  constitution.  It  consists  very 
largely  of  the  tests  of  foraminifera  and  fragments  of  shells 
of  lamellibranchs,  brachiopods  and  other  marine  calcareous 
organisms.  The  Chalk  appears  to  have  been  laid  down  in  a 
partially  enclosed  sea  of  considerable  size,  perhaps  something 
like  the  Gulf  of  Mexico.  The  Chalk  generally  contains  a  good 
many  flints  in  bands  and  nodules,  but  otherwise  it  is  remarkably 
pure,  sometimes  containing  97  or  98  per  cent,  of  calcium 
carbonate.  Hence  it  is  of  special  value  as  a  source  of  lime, 
and  is  extensively  quarried  and  burnt  for  that  purpose.  Some 
varieties  contain  a  considerable  proportion  of  phosphoric  acid, 
and  are  therefore  valuable  as  a  source  of  this  plant  food,  as 
well  as  of  lime.  The  wide  area  occupied  by  the  Chalk  in  the 
east  and  south  of  England  makes  this  formation  of  special 
importance  to  agriculture.  In  the  east  of  England,  especially 
in  Cambridgeshire,  the  lowest  division  of  the  Chalk  is  somewhat 
different  from  the  rest.  It  contains  more  argillaceous  matter, 
and  in  fact  it  forms  a  transition  between  the  limestones  and  the 
marls.  It  is  therefore  known  as  the  Chalk  Marl,  and  it  is  largely 
used  in  the  manufacture  of  Portland  cement,  since  in  it  clay 
and  carbonate  of  lime  are  already  mixed  in  the  correct  propor- 
tions. 

Marl.  This  term  is  often  used  somewhat  vaguely  by 
agricultural  writers,  but  in  the  strict  sense  it  indicates  a  clay 
with  a  considerable  proportion  of  carbonate  of  lime,  and  marl 
is  therefore  intermediate  between  clay  and  limestone.  Marls 
when  newly  formed  are  soft  and  plastic,  but  are  often  consoli- 
dated at  a  later  stage  into  argillaceous  limestones.  They  are 
very  commonly  formed  in  fresh-water  lakes,  and  in  estuaries, 
consisting  to  a  large  extent  of  broken  shells  and  calcareous 
mud.  A  marine  variety,  the  Chalk  Marl,  was  described  in  the 
last  section.  The  sites  of  the  recently  drained  meres  of  the 
fenland  are  often  occupied  by  deposits  of  shell  marl,  a  soft, 
incoherent,  white  deposit,  chiefly  composed  of  the  shells  of 
fresh- water  mollusca. 

Dolomite-rock  or  magnesian  limestone.  Most  limestones 
contain  more  or  less  magnesium  carbonate,  since  this  substance 


94  SEDIMENTS 

is  found  in  small  proportion,  not  usually  exceeding  8  or  10  per 
cent.,  in  calcareous  organisms,  but  in  some  rocks  the  mineral 
dolomite  is  an  important,  and  sometimes  almost  the  only 
constituent.  The  composition  and  characters  of  the  mineral 
dolomite  have  already  been  described  (see  p.  9).  The  origin 
of  dolomite-rock  has  given  rise  to  a  good  deal  of  controversy, 
and  even  yet  the  question  is  still  unsettled.  In  a  few  instances 
it  appears  that  the  dolomite-rock  is  an  original  formation, 
having  been  produced  directly  by  chemical  or  physical  processes, 
such  as  the  evaporation  of  solutions  rich  in  carbonates  of  lime 
and  magnesia.  But  this  direct  origin  appears  to  be  rare,  and 
in  the  majority  of  cases  it  is  quite  certain  that  the  dolomite-rock 
was  at  first  a  normal  limestone,  consisting  mainly  of  calcium 
carbonate,  with  the  usual  small  admixture  of  magnesia  and  other 
impurities.  The  conversion  of  ordinary  limestone  into  mag- 
nesian  limestone  is  known  as  dolomitization,  and  it  has  taken 
place  to  some  extent  in  many  or  most  of  the  older  calcareous 
sediments.  It  appears  to  be  specially  liable  to  occur  in  coral- 
rock,  and  this  fact  is  of  much  importance  in  connexion  with  the 
origin  of  the  change.  The  hard  parts  of  corals  consist  of 
aragonite,  a  form  of  calcium  carbonate  less  stable  than  calcite, 
and  therefore  more  liable  to  take  part  in  chemical  reactions. 
Sea  water  is  notably  rich  in  magnesium  salts,  and  it  is 
generally  believed  that  dolomite  is  formed  by  a  reaction 
between  calcium  carbonate  and  these  magnesium  salts,  especially 
magnesium  sulphate,  resulting  in  the  formation  of  dolomite 
and  calcium  sulphate.  This  theory  is  supported  by  the  common 
association  of  the  mineral  gypsum  with  beds  of  dolomite-rock. 
The  reaction  is  apparently  favoured  by  high  temperature  and 
a  pressure  of  from  one  to  five  atmospheres.  Such  conditions 
are  attained  in  tropical  regions  from  the  surface  of  the  sea  down 
to  a  depth  of  a  little  over  20  fathoms,  and  in  such  situations  it 
appears  from  observations  on  coral-reefs  that  dolomitization  is 
most  active.  In  this  way  were  probably  formed  the  famous 
"Dolomites"  of  the  Tyrol,  which  are  of  Triassic  age.  The  most 
extensive  mass  of  dolomite-rock  in  Britain  is  that  known  as 
the  Magnesian  Limestone,  of  Permian  age.  This  forms  a 
broad  band  extending  from  Nottingham  to  the  mouth  of  the 


IV] 


SEDIMENTS 


95 


Tyne,  and  reaching  a  maximum  thickness  of  800  feet  in 
Durham. 

Many  of  the  older  limestones  are  found  to  be  largely  dolomitic 
near  the  upper  surface  and  along  the  major  joints.,  but  unaltered 
in  the  inner  and  lower  parts.  It  is  often  easy  to  show  that  such 
masses  have  been  overlain  by  Triassic  deposits  containing  rock- 
salt  and  gypsum,  and  it  is  clear  in  many  instances  that  the 
dolomitization  was  effected  by  percolation  of  salt  solutions 
from  the  overlying  rocks. 

It  is  only  rarely  that  dolomitization  is  complete;  usually 
the  rock  contains  an  excess  of  calcium  carbonate  and  therefore 
consists  of  a  mixture  of  calcite  and  dolomite.  In  the  following 
table  are  given  analyses  of  some  typical  magnesian  limestones, 
British  and  foreign: 


Port  Shepstone, 
Natal  (Hatch) 

Langkofl, 
Tyrol  (Skeats) 

Newcastle, 
(Rosenbuscb) 

SiO9  etc.,  insol.   ... 

4-29 

0-02 

5-00 

Fed          
A1203        
CaCO3      

0-47) 
0-03/ 
57-75 

54-70 

1-50 
57-50 

MgC08      

37-46 

45-30 

35-33 

100-00 


100-02 


99-33 


Dolomite-rock,  when  burnt  in  a  kiln  like  limestone,  is  con- 
verted into  a  mixture  of  lime  and  magnesia.  Lime  made  from 
a  highly  magnesian  limestone  however  is  not  generally  suitable 
for  agricultural  purposes,  as  it  is  more  caustic  than  pure  lime, 
and  has  a  "burning"  effect  on  crops.  Magnesian  limestone 
is  however  employed  to  a  considerable  extent  in  certain 
metallurgical  processes. 

Ironstone.  The  ironstones  constitute  a  large  group  of 
rocks,  almost  exclusively  of  sedimentary  origin,  and  formed  in 
various  ways,  but  all  agreeing  in  containing  a  sufficiently  large 
proportion  of  iron  to  make  them  of  value  as  ores  of  that  metal. 
Iron  ores  of  purely  igneous  origin  are  rare  and  need  not  be  further 
considered  here.  The  sedimentary  ironstones  may  be  divided 
roughly  into  two  classes ;  those  which  were  ironstones  from  the 
beginning  of  their  existence,  and  those  which  were  produced 
bv  alteration  of  limestones. 


96  SEDIMENTS  [CH. 

The  formation  of  ferruginous  deposits  can  be  seen  in  progress 
at  the  present  time  in  certain  lakes  and  swamps,  where  there 
is  much  decaying  vegetation.  All  natural  waters  contain  iron 
in  solution,  chiefly  as  salts  of  various  organic  acids  of  the  humic 
group.  Under  certain  conditions  this  iron  undergoes  oxida- 
tion, forming  insoluble  hydrated  oxides,  of  which  limonite, 
2Fe203  .  3H20,  is  the  most  common.  This  oxidation  is  probably 
brought  about  by  the  action  of  bacteria1,  or  according  to  other 
authors  by  diatoms.  Perhaps  the  most  striking  example  of 
such  a  method  of  iron  ore  formation  is  afforded  by  the  "lake- 
ores"  of  Sweden,  but  a  similar  process  is  in  operation  in  many 
swamps  and  marshes  in  other  countries.  Its  chief  agricultural 
importance  lies  in  the  fact  that  in  some  wet  peaty  soils  a  hard 
layer  of  hydrated  iron  oxide  is  formed  a  few  inches  below  the 
surface  and  this  is  often  thick  and  hard  enough  to  interfere 
seriously  with  proper  cultivation.  This  is  one  form  of  "pan" 
(see  also  p.  140). 

In  the  Coal  Measures  are  found  abundantly  nodules  and  beds 
of  impure  earthy  ironstone  (clay-ironstone  and  black-band 
ironstone).  These  varieties  consist  of  carbonate  and  oxides  of 
iron  in  varying  proportions,  together  with  silica,  alumina, 
phosphoric  acid  and  other  impurities,  and  often  a  good  deal 
of  carbonaceous  material  is  present,  derived  from  the  decay 
of  organic  matter  of  animal  and  vegetable  origin.  They  are 
sometimes  very  fossiliferous.  The  origin  of  these  ironstones  is 
obviously  similar  to  that  of  the  modern  lake-ores  and  bog-ores. 

The  mineral  haematite,  Fe203 ,  one  of  the  most  valuable  ores 
of  iron,  occurs  in  mineral  veins,  and  more  commonly  in  large 
masses  in  fissures  and  cavities  in  sedimentary  rocks,  especially 
in  limestones.  Perhaps  the  best-known  occurrence  is  in  the 
Furness  district  of  Lancashire,  where  it  gives  rise  to  the  great 
iron  industry  of  Barrow-in-Furness.  Further  north,  in  west 
Cumberland,  haematite  is  also  found  in  masses  in  the  Skiddaw 
Slates,  as  well  as  in  the  Carboniferous.  It  appears  to  have 
been  deposited  mainly  by  water  holding  it  in  solution  percolating 
through  from  overlying  ferruginous  strata. 

1  Winogradsky,  "tiber  Eisenbakterien,"  Bolanische  Zeitung,  1888,  p.  260. 


iv]  SEDIMENTS  97 

Some  of  the  most  important  ironstones  in  this  country  are 
those  that  have  been  produced  by  alteration  of  limestones. 
This  change  is  in  its  simplest  form  a  replacement  of  calcium 
carbonate  (calcite  or  aragonite)  by  ferrous  carbonate  (chalybite). 
The  change  is  brought  about  by  solutions  containing  iron  salts, 
such  as  are  found  percolating  through  all  rocks.  Since  aragonite 
is  less  stable  than  calcite,  this  mineral,  if  present,  is  attacked 
first.  The  iron  carbonate  first  formed  is,  however,  itself 
unstable  and  easily  undergoes  further  oxidation  to  limonite, 
haematite  or  even  in  rare  cases  to  magnetite.  Ironstones  formed 
in  this  way  are  often  highly  fossiliferous,  and  show  all  the 
structures  characteristic  of  limestones,  especially  the  oolitic 
structure.  The  best  examples  in  this  country  are  the  ironstones 
of  the  Jurassic  system  in  the  Midlands  and  in  Yorkshire.  The 
Cleveland  Main  Seam,  in  the  Middle  Lias,  is  as  much  as  25  feet 
thick  at  Eston,  near  Middlesborough,  and  has  given  rise  to  an 
enormous  mining  industry  in  the  Cleveland  area.  The  Top  Seam 
or  Dogger  of  the  same  district  is  of  much  inferior  quality,  but 
extends  over  a  very  large  area.  At  about  the  same  horizon 
(the  base  of  the  Inferior  Oolite)  are  the  well-known  ironstones 
of  the  Northampton  Sands  series  of  Lincoln,  Rutland  and 
Northampton.  Ironstone  is  also  worked  in  the  Lower  Lias  at 
Frodingham  and  Scunthorpe  in  Lincolnshire.  In  times  past  a 
great  iron-smelting  industry  also  existed  in  the  Weald  of  Sussex 
and  Kent,  where  wood  was  used  for  fuel,  but  on  the  failure  of 
the  wood  supply  the  workings  were  abandoned. 

The  presence  of  beds  of  ironstone  is  not  directly  of  much 
agricultural  importance,  but  indirectly  it  has  one  useful  result. 
Ironstones  are  often  comparatively  rich  in  phosphoric  acid, 
and  weathered  material  distributed  over  the  soil  by  rain-wash 
from  an  outcrop  at  a  higher  level  may  be  a  useful  natural 
source  of  this  constituent.  Such  is  found  to  be  the  case  with 
some  of  the  soils  in  the  neighbourhood  of  the  Northampton 
ironstones.  Again  perhaps  the  cheapest  and  most  convenient 
of  all  "artificial"  phosphatic  manures  is  basic  slag,  which  is  a 
by-product  in  the  manufacture  of  steel  from  phosphatic  iron- 
stone. 

In   many  countries,   though  not  in   Britain,   parts  of  the 

R.  A.G  7 


98  SEDIMENTS  [CH. 

crystalline  schists  of  the  Archaean  systems  are  highly  ferru- 
ginous, often  being  very  rich  in  haematite  or  magnetite.  This 
appears  to  be  due  largely  to  secondary  enrichment  by  solutions 
either  before  or  during  metamorphism.  Good  instances  are 
afforded  by  the  Penokee  iron-bearing  schists  of  Michigan,  and 
the  quartz-haematite  and  quartz-magnetite  schists  of  the 
Swaziland  system  in  South  Africa.  The  latter  are  curious 
rocks,  showing  conspicuous  bands  of  white  and  black  or  red 
(the  "calico-rock"  of  the  miners  and  prospectors).  Although 
rich  in  iron,  such  rocks  are  generally  too  siliceous  to  be  worked 
profitably  as  ores. 

Silicification  of  limestones.  Flint  and  chert.  It  has  often  been 
observed  that  calcium  carbonate  and  silica  are  able  to  replace 
each  other  in  rocks,  the  process  going  on  sometimes  in  one 
direction,  sometimes  in  the  other.  Most  commonly  however 
calcium  carbonate  is  replaced  by  silica,  and  the  rearrangement 
of  the  silica  originally  in  the  sediment  as  well  as  addition  of 
this  substance  from  outside,  gives  rise  to  some  very  well-known 
rock- types ;  of  these  flint  and  chert  may  be  specially  mentioned. 

The  simplest  case  of  all  is  where  the  aragonite  or  calcite  of 
a  limestone  has  been  replaced  molecule  for  molecule  by  silica, 
preserving  perfectly  all  the  structures  of  the  limestone,  organic 
and  otherwise,  these  now  consisting  of  chalcedony  instead  of 
the  original  mineral.  A  well-known  example  is  to  be  found  in 
the  cherts  of  the  Portland  series,  which  often  show  very  perfectly 
preserved  masses  of  reef-corals.  In  this  case  the  silica  must 
have  been  brought  in  from  outside.  More  commonly  however 
chert  is  formed  by  rearrangement  of  silica  already  existing  in 
the  sediment,  very  often  in  the  form  of  sponge  remains.  The 
skeletons  of  most  sponges  consist  of  colloidal  silica,  which 
is  more  readily  soluble  than  the  crystalline  form.  This  is 
dissolved  by  percolating  water  and  often  deposited  again 
almost  immediately  in  the  solid  state,  sometimes  in  a  con- 
cretionary form,  around  some  originally  siliceous  nucleus ; 
sometimes  in  a  more  than  usually  siliceous  layer  or  bed.  It 
appears  that  silica  tends  to  travel  towards  points  of  more  than 
the  average  concentration  and  any  unusually  siliceous  patch 
will  therefore  act  as  a  centre  for  deposition.  Some  of  the  most 


iv]  SEDIMENTS  99 

highly  siliceous  bands  of  the  Lower  and  Upper  Greensand 
seem  to  have  been  largely  composed  of  remains  of  sponges, 
and  these  originally  very  siliceous  beds  underwent  secondary 
enrichment,  forming  well-marked  layers  of  chert.  The  same 
remark  applies  to  the  cherts  of  the  Carboniferous  Limestone 
in  west  Yorkshire  and  North  Wales;  these  are  of  some  com- 
mercial value  in  the  manufacture  of  certain  kinds  of  pottery.  • 

The  best-known  siliceous  concretions  are  the  flints,  found 
originally  in  the  Chalk,  but  now  forming  such  an  important 
constituent  of  nearly  all  the  later  formations  of  the  south  and 
east  of  England.  Owing  to  their  stability  and  great  hardness 
flints  are  passed  on  from  one  formation  to  another  with  little 
change,  except  a  slight  reduction  in  size  by  mechanical  wear. 
Flints  vary  much  in  colour,  from  black,  though  every  shade  of 
grey,  brown  and  yellow  to  nearly  white ;  when  freshly  dug  out 
of  the  Chalk  they  are  generally  covered  with  a  thin  white  film, 
but  this  is  soon  lost  on  weathering  or  rubbing.  The  variations 
in  size  and  shape  are  endless,  but  generally  quite  irregular 
and  more  or  less  nodular.  Flints  may  be  described  briefly  as 
concretionary  masses  of  cryptocrystalline  (i.e.  very  finely 
crystalline)  silica,  formed  by  concentration  of  silica,  formerly 
more  or  less  disseminated  through  the  Chalk,  around  certain 
points  or  along  certain  planes.  The  core  of  a  flint  is  often  a 
sponge  or  other  fossil,  and  the  so-called  tabular  flints  may  be 
formed  either  along  bedding- planes  or  along  joint-planes 
inclined  to  the  bedding.  From  a  study  of  the  microscopic 
characters  of  Chalk  it  appears  that  the  silica  of  the  flints  is 
obtained  from  sponges,  radiolaria,  diatoms  and  other  organisms 
originally  forming  part  of  the  Chalk  itself. 

There  appears  to  be  no  essential  difference  between  flint  and 
chert;  it  is  simply  a  matter  of  nomenclature,  the  fact  being 
that  the  characteristic  siliceous  bodies  from  the  Chalk  are 
universally  called  flints,  while  siliceous  masses  from  all  other 
formations,  whether  of  similar  or  different  origin,  are  called 
chert.  It  is  hardly  necessary  to  point  out  here  the  important 
part  played  by  flint  as  a  material  for  tools  and  weapons  in  the 
early  stages  of  human  development.  This  subject,  which  is  of 
the  greatest  interest,  belongs  to  pre-historic  archaeology  rather 

7—2 


100  SEDIMENTS  [CH. 

than  to  geology.  The  farmer  is  concerned  only  with  flints  as 
constituents  of  soils  and  of  superficial  deposits  in  general,  and 
sometimes  as  road-material;  for  this  latter  purpose  they  are 
very  badly  suited  owing  to  their  hardness  and  sharpness ;  they 
are  specially  destructive  to  rubber  tyres. 

Phosphatization  and  phosphatic  deposits.  The  great  agri- 
cultural importance  of  phosphoric  acid  as  a  constituent  of 
soils  and  of  manures  and  as  a  plant  food  necessitates  a 
somewhat  full  account  of  the  origin  and  characters  of  naturally 
occurring  phosphatic  deposits. 

The  ultimate  source  of  phosphorus  compounds  in  sediments 
and  in  soils  is  to  be  sought  in  the  mineral  apatite,  a  widely 
distributed  constituent  of  igneous  rocks  of  all  kinds  and  of  all 
ages.  In  a  few  places,  as  for  example  in  south-western  Norway, 
apatite  occurs  in  workable  quantities  in  veins  and  segregations 
in  igneous  rock  (gabbro  or  dolerite).  In  other  parts  of 
Norway,  in  Spain  and  in  Canada  great  masses  of  crystallized 
apatite  are  found;  these  are  believed  to  have  been  originally 
of  sedimentary  origin  and  afterwards  crystallized  by  meta- 
morphism.  All  of  these  are  now  largely  employed  in  the 
manufacture  of  superphosphate.  For  all  practical  purposes 
apatite  may  be  regarded  as  pure  calcium  phosphate ;  the  small 
amount  of  fluorine  or  chlorine  present  in  the  mineral  is  of  no 
agricultural  significance;  consequently  superphosphate  made 
from  apatite  is  generally  of  very  high  grade. 

When  a  rock  containing  apatite  is  weathered  the  apatite 
appears  to  dissolve,  as  it  is  very  rarely  found  in  the  form  of 
crystals  in  detrital  sediments.  Thus  the  calcium  phosphate 
gets  into  circulation  in  natural  waters,  and  sets  up  an  endless 
cycle  of  chemical  and  physiological  processes,  passing  into  the 
sediments  and  soils,  from  thence  into  plants  and  ultimately  into 
the  bones  and  tissues  of  animals.  The  excreta  of  animals  and 
the  decay  of  their  bodies  after  death  continue  the  process  to 
further  stages,  and  the  whole  story  is  evidently  of  very  great 
complexity.  The  main  point  is  that  natural  waters  always 
contain  soluble  compounds  of  phosphoric  acid.  These  are  in 
part  taken  up  directly  by  plants  and  animals,  the  other  part 
promoting  chemical  changes  in  rocks  and  soils.  It  is  clear  that 


iv]  SEDIMENTS  101 

when  such  phosphatic  solutions  come  in  contact  with  carbonates, 
the  latter  are  decomposed  and  phosphoric  acid  is  substituted 
for  the  carbonic  acid,  forming  insoluble  phosphates. 

The  study  of  deposits  of  guano  and  of  the  rocks  underlying 
them  has  thrown  much  light  on  this  subject.  Guano  is  very 
rich  in  soluble  phosphate,  probably  existing  to  a  large  extent  as 
ammonium  phosphate  and  this  is  often  carried  down  into  the 
rocks  below.  When,  as  often  happens,  the  guano  lies  on  coral- 
rock,  it  is  often  found  that  the  calcium  carbonate  is  more  or 
less  completely  changed  to  calcium  phosphate.  It  is  believed 
that  the  great  phosphate  deposits  of  Christmas  Island,  in  the 
Indian  Ocean,  were  formed  by  this  means,  although  the  guano 
has  now  completely  disappeared.  Cases  are  also  known  where 
igneous  rocks  have  been  converted  into  phosphates  of  iron  and 
alumina  by  a  similar  process. 

Where  deposits  of 'guano  have  been  leached  out  by  rain, 
most  of  the  nitrogenous  matter,  being  easily  soluble,  is  removed, 
and  the  less  soluble  phosphates  are  left  behind.  Furthermore, 
since  calcium  phosphate  is  more  soluble  than  phosphates  of 
iron  and  aluminium,  excessive  washing  will  remove  the  former 
and  leave  the  latter  behind.  Hence  some  naturally  washed 
guano  residues,  where  the  leaching  process  has  not  gone  too 
far,  are  good  phosphatic  manures  in  their  original  state,  but  if 
little  but  iron  and  alumina  compounds  are  left  the  manurial 
value  is  small.  In  an  analysis  of  a  natural  phosphate  it  is 
insufficient  and  misleading  to  determine  only  the  total  phos- 
phoric acid;  it  should  also  be  stated  what  proportion  of 
this  is  present  as  calcium  phosphate,  since  this  alone  is  of  much 
value. 

All  the  larger  natural  deposits  of  calcium  phosphate,  or 
phosphorite,  are  probably  of  marine  origin  and  they  are 
apparently  always  due  directly  or  indirectly  to  organic  agencies. 
Unlike  apatite,  they  are  amorphous  and  may  be  either  compact, 
earthy  or  concretionary.  Nodular  forms  are  very  common  in 
rocks  of  many  ages,  and  have  been  dredged  up  from  consider- 
able depths  among  the  deposits  of  the  modern  sea-floor,  nearly 
always  in  association  with  the  mineral  glauconite.  They  must 
therefore  be  regarded  as,  at  any  rate  in  part,  original  constituents 


SEDIMENTS 


CH. 


of  sediments.  But  it  is  clear  that  many  phosphatic  nodules 
were  originally  calcareous  and  'only  became  phosphatic  at  a 
later  stage.  Such  were  the  co-called  "  coprolites1 "  of  the 
Cretaceous  strata  of  the  south  and  east  of  England,  and  many 
others  of  similar  origin.  The  same  statement  is  true  of  many 
phosphatic  beds,  layers  and  masses  in  rocks  of  various  ages. 
In  many  pebble-beds  of  shallow-water  marine  origin,  and 
especially  in  those  associated  with  local  unconformities  and 
"wash-outs"  there  are  to  be  found  phosphatic  nodules  of  a 
special  character,  nearly  always  accompanied  by  glauconite. 
Many  of  the  phosphatic  nodules  (coprolites  of  the  trade)  are 
obviously  rolled  fossils,  or  fragments  of  fossils,  derived  from 
older  rocks  during  the  concurrent  denudation;  besides  these 
there  are  often  many  concretionary  masses  of  phosphate  of  the 


,.       _ 

i 

I          -  1 

1 

i 

nr~ 

Fig.  30.     Mode  of  occurrence  of  the  Cambridge  Greensand. 
a.  Lower  Greensand;   b,  Gault;  c,  Cambridge  Greensand;  d,  Chalk. 

type  before  described.  The  fossils  must  have  been  originally 
calcareous  and  they  have  been  phosphatized  while  lying 
uncovered  on  the  sea-floor.  Shallow  seas,  especially  where 
wave-action  and  currents  prevail,  always  abound  in  animal  life, 
and  the  phosphoric  acid  is  derived  from  the  excreta  and  from 
the  decaying  remains  of  these  animals,  among  which  fish,  and 
in  past  ages  gigantic  reptiles,  are  the  most  prominent.  The  best 
example  of  this  process  is  afforded  by  the  well-known  Cambridge 
Greensand,  which  30  or  40  years  ago  gave  rise  to  an  important 

1  The  term  coprolite  has  been  much  misapplied.  Originally  it  meant  the 
fossil  excreta  of  animals,  but  it  was  later  used  as  a  commercial  name  for  any 
kind  of  phosphatic  nodules,  of  whatever  origin. 


iv]  SEDIMENTS  103 

industry.  Owing  to  a  slight  local  disturbance  a  small  uncon- 
formity was  produced  between  the  Gault  and  the  Chalk ;  most  of 
the  finer  material  of  the  Upper  Gault  was  washed  away,  the 
pebbles,  nodules  and  fossils  remaining  behind  to  be  phosphatized. 
Many  of  the  fossils  of  the  Gault  were  already  more  or  less 
phosphatic  and  underwent  a  secondary  enrichment,  so  that  their 
percentage  of  phosphoric  acid  is  now  very  high.  The  animals 
which  were  living  at  the  time  and  were  entombed  in  the  Green- 
sand  also  underwent  a  lower  grade  of  phosphatization.  These 
can  be  distinguished  by  their  paler  colour.  Of  somewhat 
similar  origin  and  character  are  the  phosphatic  nodule  beds  of 
the  Lower  Greensand,  formerly  worked  at  various  localities  in 
Bedfordshire  and  Cambridgeshire.  At  various  horizons  in  the 
Jurassic  and  Cretaceous  clay-formations  there  are  layers  of 
more  or  less  phosphatic  nodules,  and  these  have  undoubtedly 
in  part  been  the  source  of  the  concretionary  nodules  of  the 
"derived"  pebble  beds  just  described;  with  them  may  be 
compared  the  layers  of  phosphatic  concretions  in  the  Ecca 
shale  near  Ladysmith.  These  are  associated  with  fish-remains, 
which  were  no  doubt  the  source  of  the  phosphoric  acid. 

The  frequent  association  of  phosphates  and  glauconite  has 
already  been  mentioned,  and  this  is  a  point  of  some  importance ; 
glauconite  is  a  conspicuous  mineral,  easy  to  detect  owing  to  its 
green  colour,  and  its  presence  affords  a  useful  indication  of  the 
possible  existence  in  the  same  deposit  of  useful  supplies  of 
phosphate. 

Calcium  phosphate  is  a  constituent  of  some  varieties  of 
Chalk,  and  its  presence  is  of  considerable  importance,  since  lime 
made  from  phosphatic  Chalk  obviously  serves  a  double  purpose, 
enriching  the  soil  in  phosphoric  acid  as  well  as  in  lime.  In 
some  varieties  the  phosphate  appears  to  be  finely  disseminated 
throughout  the  rock,  while  in  others  it  occurs  as  white,  grey  or 
yellowish  nodules,  apparently  of  concretionary  origin,  as  in 
parts  of  the  Lower  Chalk  of  Cambridgeshire.  Phosphatic 
Chalk  has  been  worked  commercially  on  a  considerable  scale  at 
various  places  in  the  north  of  France,  in  the  departments  of  the 
Somme,  Oise  and  Nord,  and  especially  at  Ciply  in  Belgium. 
The  workings  at  the  latter  place  yielded  85,000  tons  of  phosphate 


104  SEDIMENTS  [CH. 

in  1884.  The  Ciply  Chalk  is  greyish-brown  in  colour,  and  the 
phosphate  exists  in  the  form  of  minute  brown  grains,  all  of 
which  can  be  referred  to  an  organic  origin,  many  being  casts  of 
Foraminifera.  A  bed  of  phosphatic  Chalk  of  very  similar  char- 
acter exists  at  Taplow.  Here  also  the  phosphate  is  in  the  form 
of  brown  grains  and  fragments,  many  of  these  being  evidently 
bits  of  scales  and  teeth  of  fish,  as  well  as  Foraminifera  and  shell 
fragments,  the  latter  having  evidently  undergone  secondary 
phosphatization,  as  in  the  Cambridge  Greensand.  Some  oval 
pellets  appear  to  be  excreta  of  fish  and  doubtless  much  of  the 
phosphate  is  derived  from  this  source. 

The  following  analyses,  somewhat  condensed  from  the 
original  publication1,  show  the  composition  of  samples  of 
phosphatic  Chalk  from  Ciply  and  from  Taplow. 

Ciply  Taplow 

Organic  matter  and  moisture     ...  2-83  '     .  3-0 

Lime           53-24  53-7 

Iron  oxide  and  alumina  ...         ...  1-01  -9 

Potash  and  soda              -19  -3 

Carbon  dioxide 28-10  28-7 

Phosphoric  acid 11-66  11-6 

Silica          1-96  -5 

The  similarity  in  composition  is  here  very  striking.  The 
Ciply  phosphate  is  concentrated  by  mechanical  processes  or  by 
washing,  and  the  natural  weathered  product  found  in  pockets 
in  the  rock  contains  up  to  67  per  cent,  of  calcium  phosphate, 
the  more  soluble  carbonate  having  been  removed  by  percolating 
water.  This  is  obviously  a  valuable  fertilizer. 

Salt  deposits.  These  constitute  a  peculiar  and  well-marked 
class  of  aqueous  accumulations,  some  of  which  are  of  the  very 
highest  agricultural  importance ;  as  examples  may  be  mentioned 
rock-salt,  kainite  and  nitrate  of  soda.  Although  for  the  most 
part  of  purely  chemical  origin,  they  are  as  a  matter  of  practical 
convenience  classed  with  the  sediments  by  nearly  all  authors. 
All  the  substances  here  treated  together  agree  in  the  fact  that 
they  are  easily  soluble  in  water,  and  therefore  their  accumulation 
in  the  solid  form  can  only  take  place  under  special  conditions, 

1  Strahan,  "On  a  phosphatic  Chalk... at  Taplow,"  Quart.  Journ.  Geol.  Soc. 
vol.  XLVII.  1891,  p.  356,  and  vol.  LIT.  1896,  p.  463. 


iv]  SEDIMENTS  105 

the  most  important  of  these  conditions  being  absence  of  rainfall. 
Hence  salt-deposits  are  specially  characteristic  of  arid  regions. 
It  is  only  in  the  desert  zone  that  the  requisite  climatic  conditions 
are  attained. 

All  natural  waters  contain  more  or  less  dissolved  mineral 
matter,  and  in  fact  the  distinction  between  fresh  and  salt  water 
is  merely  one  of  degree ;  the  water  is  called  salt  when  it  contains 
enough  dissolved  matter  to  impart  a  distinct  taste;  when 
tasteless  it  is  called  fresh.  Now  when  a  salt  solution  is  evapor- 
ated the  water  is  driven  off  in  the  form  of  vapour,  and  the  salt 
remains  behind.  A  salt  solution,  however  dilute  at  first,  can 
be  concentrated  by  evaporation,  and  fresh  water  thus  becomes 
salt.  Such  a  process  occurring  naturally  on  a  large  scale  has 
given  rise  to  most  of  the  salt-deposits  known  to  us.  the  chief 
exception  being  in  those  instances  where  masses  of  sea-water 
may  become  isolated  and  dry  up. 

The  process  last  mentioned  sometimes  occurs  on  a  compara- 
tively small  scale  in  lagoons  along  the  sea-coast;  owing  to 
slight  earth-movement,  or  to  growth  of  a  shingle  bar,  or  some 
similar  cause,  communication  with  the  sea  is  cut  off,  and  if  the 
climate  is  warm  enough  the  water  will  dry  up,  leaving  the  salts 
behind.  An  artificial  adaptation  of  this  process  is  employed 
in  some  warm  regions  for  the  manufacture  of  salt,  as  for  example 
in  the  salt  pans  on  the  Mediterranean  coast  of  France.  On  a 
very  large  scale  the  formation  of  certain  inland  seas,  such  as 
the  Caspian,  is  an  exactly  analogous  process.  At  one  time  the 
area  now  occupied  by  the  Caspian,  the  Sea  of  Aral  and  other 
salt  lakes  of  south-western  Asia  was  in  free  and  open  communi- 
cation with  the  Arctic  Ocean.  As  a  result  of  earth  movements 
this  communication  was  cut  off,  and  the  whole  area  converted 
into  a  self-contained  closed  drainage  basin  with  no  outlet  to 
the  sea.  A  little  consideration  will  show  that  the  maintenance 
of  such  a  condition  must  depend  upon  the  ratio  between  loss 
and  gain  of  water ;  that  is  to  say,  if  the  rainfall  in  the  drainage 
area  exceeds  the  loss  by  evaporation  the  basin  will  gradually 
fill  up  with  water  till  an  outlet  is  established  at  the  lowest 
point  on  the  rim ;  in  course  of  time  all  the  salt  will  be  washed 
out  and  the  lake  will  become  fresh.  On  the  other  hand,  if 


106  SEDIMENTS  [CH. 

evaporation  is  in  excess  the  lake  will  become  salter  and  salter, 
till  the  water  is  saturated  and  salts  are  deposited. 

But  a  salt  lake  may  also  originate  without  any  connexion 
with  the  sea  at  any  period  of  its  existence.  If  by  some  means 
a  fresh-water  lake  loses  its  outlet,  it  will  gradually  become  salt 
on  account  of  the  dissolved  mineral  matter  brought  in  by  rivers, 
and  the  waters  of  the  lake  will  eventually  reach  saturation 
point.  Lakes  without  outlet  are  common  in  arid  regions,  in 
fact  wherever  evaporation  exceeds  rainfall,  and  it  is  in  this  way 
that  most  of  the  well-known  salt  lakes  have  been  formed,  for 
example,  the  Dead  Sea,  and  the  Great  Salt  Lake  of  Utah. 
Both  are  relics  of  once  much  larger  fresh-water  lakes,  which 
have  nearly  dried  up,  and  the  formation  of  the  Dead  Sea  was 
specially  favoured  by  the  fact  that  the  rocks  of  the  Jordan 
valley  were  originally  very  rich  in  salt.  This  is  also  a  region  of 
little  rainfall  and  very  high  temperature. 

When  a  mixed  salt-solution,  such  as  sea- water,  is  evaporated, 
the  salts  are  deposited  in  a  certain  definite  order,  depending 
on  the  relative  solubilities  of  the  salts  present.  The  first  salt 
formed  from  natural  waters  is  calcium  sulphate,  which  is 
deposited  as  gypsum  or  anhydrite  according  to  temperature,  the 
former  being  the  more  common.  This  is  followed  by  sodium 
chloride  (common  salt)  while  the  salts  of  potassium  and  mag- 
nesium, being  much  more  soluble,  remain  in  solution  till  nearly 
all  the  water  is  evaporated,  and  it  is  only  in  very  rare  cases  that 
they  have  been  deposited  in  nature.  The  laws  governing  the 
crystallization  of  potassium  and  magnesium  salts  are  very 
complex,  and  a  large  number  of  minerals  have  been  formed  by 
this  process,  the  most  important  from  an  agricultural  point  of 
view  being  kainite  and  carnallite. 

The  best  example  of  what  is  known  as  a  "complete"  salt 
deposit  (that  is.  one  including  potassium  and  magnesium  salts) 
is  found  at  Stassfurt  in  Germany,  and  this  is  now  the  chief 
source  of  artificial  potash  manures. 

The  chief  salts  found  at  Stassfurt  are  enumerated  in  the 
following  table : 


iv]  SEDIMENTS  107 

Rock-salt  ...  NaCl. 

Sylvine KC1. 

Carnallite  ,..  KC1 .  MgCl2  .  6H2O. 

Polyhalite  . . .  K2SO4  .  MgSO4  .  2CaSO4  .  2H2O. 

Kainit  3  ...  KC1 .  MgSO4  .  3H,O. 

Gypsum  ...  CaSO4 .  2H2O. 

.Anhydrite  ...  CaSO4. 

In  the  lowest  part  of  the  shaft  is  about  700  feet  of  rock-salt ; 
then  comes  the  polyhalite  region,  about  200  feet  thick.  This  is 
succeeded  by  about  200  feet  of  magnesium  sulphate  with  some 
carnallite,  while  the  highest  layer,  about  100  feet  thick,  contains 
kainite,  carnallite  and  magnesium  chloride.  Resting  on  the 
salt-deposit  is  a  bed  of  impervious  clay,  and  to  this  is  doubtless 
due  the  preservation  of  all  these  highly  soluble  minerals. 
Kainite,  carnallite  and  other  potash  salts  are  also  found  at 
Kalusz  and  Aussee  in  Austria. 

All  the  evidence  indicates  that  these  salts  were  formed  by 
the  evaporation  of  enormous  volumes  of  sea-water,  although  it 
is  not  quite  clear  how  the  actual  evaporation  was  brought 
about;  whether  in  a  closed  basin  or  in  an  area  still  in  com- 
munication with  the  open  sea.  Space  will  not  permit  of  a 
full  discussion  of  this  interesting  subject1. 

Deposits  of  rock-salt  and  gypsum,  without  salts  of  potassium 
and  magnesium,  are  found  among  sediments  of  both  ancient 
and  modern  date  in  many  parts  of  the  world ;  in  fact  they  are 
highly  characteristic  of  the  desert  facies  of  deposition,  and  have 
evidently  been  formed  for  the  most  part  in  salt  lakes  without 
any  connexion  with  the  sea. 

As  an  example  we  may  take  the  Trias  salt-beds  of  the 
British  Isles ;  these  are  found  in  Cheshire,  Worcestershire,  near 
Middlesborough  in  north-east  Yorkshire,  and  in  the  neighbour- 
hood of  Belfast  (Carrickfergus).  The  salt  and  gypsum  are 
interstratified  with  beds  of  fine-grained  red  marls  and  red  and 
yellow  sandstones,  the  latter  often  showing  the  conspicuously 
rounded  grains  that  are  so  characteristic  of  desert  sands  (millet- 
seed  sands).  The  so-called  marls  are  not  generally  real  marls, 
that  is,  they  are  not  markedly  calcareous,  but  are  simply 

1  For  further  details  and  references  see  Hatch  and  Rastall,  The  Petrology 
of  the  Sedimentary  Rocks,  London,  1913,  pp.  93-108. 


108  SEDIMENTS  [CH. 

sediments  of  unusually  fine  texture ;  they  were  formed  by  dust 
drifting  before  the  wind  and  accumulating  in  salt  lakes,  in 
which  the  gypsum  and  rock-salt  were  also  deposited  by  con- 
centration of  the  saline  water. 

The  greatest  known  thickness  of  rock-salt  was  found  in  a 
deep  boring  at  Sperenberg,  near  Berlin;  this  passed  through 
about  4000  feet  of  salt  without  reaching  the  base.  Very  thick 
deposits  are  also  known  at  Wieliczka  in  Austrian  Poland,  and 
at  Parajd  in  Transylvania;  these  are  of  Tertiary  age. 

Modern  salt  lakes  can  be  conveniently  divided  on  chemical 
grounds  into  three  groups,  as  follows: 

(a)  Salt  lakes  proper,  such  as  the  Dead  Sea  and  the  Great 
Salt  Lake  of  Utah;  these  contain  chiefly  chlorides  of  sodium 
and  magnesium;  they  are  saline  to  the  taste,  but  not  bitter. 

(6)  Bitter  lakes;  these  owe  their  peculiar  taste  to  the 
abundance  of  sulphates,  especially  sodium  sulphate  (Glauber 
salt)  and  magnesium  sulphate  (Epsom  salts).  Some  well-known 
bitter  lakes  on  the  Isthmus  of  Suez  were  destroyed  by  the 
construction  of  the  Suez  canal.  They  also  occur  in  the  Aralo- 
Caspian  basin  and  in  western  America. 

(c)  Soda-lakes  or  alkali  lakes;  these  contain  abundance  of 
sodium  carbonate  and  sodium  bicarbonate.  The  natron  lakes 
of  the  Egyptian  desert  are  of  this  nature  and  others  are  found 
in  the  Great  Basin  of  western  America. 

Nitrates.  Nitrates  are  formed  in  all  fertile  soils  by  the 
action  of  certain  specific  bacteria  on  nitrogenous  organic 
matter,  and  a  small  amount  of  nitric  acid  is  also  brought  down 
from  the  atmosphere  by  rain.  Nitrates  are  highly  soluble  and 
are  either  taken  up  at  once  by  plants  or  carried  away  in  the 
drainage  water.  It  is  but  rarely  and  only  under  special  circum- 
stances that  nitrates  can  accumulate  in  the  solid  form.  Nitrate 
of  lime  is  found  in  considerable  quantity  in  certain  caves  in 
Kentucky  and  Indiana,  and  it  is  believed  to  be  formed  by 
oxidation  of  guano.  Potassium  nitrate  (saltpetre)  has  been 
observed  as  an  efflorescence  on  the  surface  of  the  soil  in  certain 
dry  regions  in  north-western  India.  But  the  chief  commercial 
and  agricultural  substance  of  this  class  is  nitrate  of  soda  or 
Chile  saltpetre,  which  is  found  in  great  masses  in  certain  parts 


ivj  SEDIMENTS  109 

of  South  America.  It  has  also  been  noticed  in  association  with 
boron  minerals  in  southern  California.  The  nitrate  beds  of 
South  America  are  found  in  the  rainless  district  of  Peru  and 
Chile,  especially  in  the  deserts  of  Atacama  and  Tarapaca, 
these  being  perhaps  the  driest  regions  of  the  world.  They  are 
often  at  a  considerable  distance  from  the  coast,  even  as  much 
as  180  miles,  and  at  considerable  elevations,  up  to  5000  feet 
above  sea-level.  These  facts  seem  to  militate  against  a  marine 
origin,  as  will  be  mentioned  later. 

The  crude  sodium  nitrate,  called  caliche,  is  interstratified 
with  sediments  of  various  kinds  in  association  with  beds  of 
common  salt  and  borax ;  the  following  is  a  section  of  a  typical 
"calichera" : 

Sand  and  gravel 2  inches. 

Gypsum       6       „ 

Compact  earth  and  stones  10  feet. 

Caliche         5     „ 

Clay  below. 

Sometimes  the  bed  of  caliche  is  as  much  as  12  feet  thick  and 
the  details  vary  slightly  in  different  localities. 

The  caliche  is  by  no  means  pure  and  it  varies  widely  in 
composition ;  the  chief  impurities  are  sodium  chloride,  sodium 
sulphate,  calcium  sulphate  and  magnesium  sulphate,  together 
with  a  small  proportion  of  borates,  iodates  and  ammonia 
compounds.  Some  samples  of  caliche  contain  a  little  potassium 
perchlorate,  a  salt  which  is  supposed  to  be  very  injurious  to 
vegetation;  such  samples  should  be  avoided. 

Many  suggestions  have  been  made  as  to  the  origin  of  the 
nitrate  deposits ;  the  following  are  some  of  the  less  improbable 
theories.  Ochsenius1  supposed  that  the  basis  of  the  whole 
process  was  to  be  found  in  beds  of  rock-salt  existing  in  the 
Andes ;  these  were  dissolved  by  rain  and  carried  down  to  lower 
levels,  where  the  solution  was  acted  on  by  carbon  dioxide  of 
volcanic  origin,  forming  sodium  carbonate.  This  is  then  de- 
composed by  dust  containing  nitrogen  and  ammonia  carried  by 
the  wind  from  beds  of  guano  on  the  coast,  forming  nitric  acid 
and  eventually  sodium  nitrate.  This  cause  seems  inadequate 
1  Ochsenius,  Die  Bildung  des  Natronsalpeters,  Stuttgart,  1887. 


110  SEDIMENTS  [CH.  iv 

to  produce  the  observed  results.  Another  somewhat  similar 
theory3  supposes  that  beds  of  guano  have  undergone  bacterial 
decomposition,  forming  nitric  acid,  and  then  calcium  nitrate, 
which  reacted  with  sodium  sulphate  in  the  underlying  strata, 
forming  sodium  nitrate  and  gypsum,  thus : 

Ca(N03)2  +  Na2S04  =  CaS04  +  2NaN03. 

It  is  certainly  a  fact  that  gypsum  is  found  along  with  the 
caliche,  but  the  absence  of  phosphorus  compounds  seems  to 
disprove  an  origin  from  guano,  in  which  phosphates  are  always 
present  in  large  quantity. 

An  older  theory  is  that  of  Noellner,  who  referred  the  origin 
of  the  nitrate  to  the  decomposition  of  great  masses  of  seaweed, 
forming  nitric  acid,  the  rest  of  the  process  being  as  last  described. 
This  accounts  satisfactorily  for  the  presence  of  iodine  and  the 
absence  of  phosphate,  but  the  chief  objection  is  the  great 
elevation  and  distance  from  the  sea  at  which  the  deposits  are 
now  found,  implying  an  enormous  rise  of  the  land  in  recent 
times.  It  has  long  been  known,  especially  from  Darwin's 
observations,  that  the  west  coast  of  South  America  is  undergoing 
elevation  even  now,  but  the  movements  postulated  seem  too 
great  to  be  credible. 

Owing  to  the  association  of  boron  salts  with  nitrate  of 
soda,  both  in  Chile  and  in  California  it  has  been  suggested  that 
these  deposits  are  ultimately  of  volcanic  origin.  This  suggestion 
is  plausible,  but  in  the  present  state  of  knowledge  not  fully 
established.  We  must  be  content  with  the  statement  that  the 
origin  of  nitrate  of  soda  is  as  yet  an  unsolved  problem. 

1  Plagemann,  Die  Dungstoffindustrie  der  Welt,  Berlin,  1904. 


CHAPTER   V 

SUPERFICIAL  DEPOSITS 

Of  the  highest  importance  from  the  agricultural  point  of 
view  are  the  superficial  accumulations  of  varying  origin  which 
in  so  many  places  mask  the  solid  rocks,  and  by  their  weathering 
and  alteration  give  rise  to  the  soil  and  subsoil.  In  this  chapter 
the  actual  cultivated  layer  of  the  soil  is  not  dealt  with,  and 
attention  is  confined  to  the  origin  and  characters  of  the  deposits 
of  recent  origin  and  very  varying  thickness  that  constitute  the 
subsoil  in  many  regions.  These,  when  present,  obviously  have 
a  controlling  influence  in  determining  the  agricultural  value  of 
the  soil,  and  their  study  is  of  the  utmost  importance  to  the 
farmer. 

For  these  deposits  many  classifications  have  been  proposed, 
but  none  are  wholly  satisfactory,  partly  owing  to  the  infinite 
variety  of  the  deposits,  and  partly  because  apparently  similar 
accumulations  may  have  been  formed  in  very  different  ways. 
The  classification  here  adopted  is  based  upon  that  proposed  by 
Merrill1,  but  certain  modifications  have  been  introduced,  in 
order  to  render  the  treatment  more  suitable  for  the  special 
purpose  of  this  book,  and  the  original  American  terminology 
has  been  in  some  instances  replaced  by  terms  better  understood 
in  England. 

The  superficial  deposits  may  be  divided  primarily  into 
two  broad  groups,  namely,  sedentary  and  transported.  The 
sedentary  deposits  are  formed  mainly  by  the  weathering  and 
alteration  of  the  underlying  rocks,  and  their  character  is 
determined  entirely  by  the  nature  of  these.  Some  also  are 

1  Merrill,  A  Treatise  on  Rocks,  Rock-weathering  and  Soils,  New  York,  1897, 
p.  300. 


112  SUPERFICIAL  DEPOSITS  [CH. 

formed  wholly  or  in  part  by  the  growth  and  decay  of  vegetation. 
Transported  deposits  on  the  other  hand  have  been  formed  of 
material  brought  from  a  greater  or  less  distance  by  the  various 
geological  agents:  gravity,  water,  ice  and  wind.  They  need 
show  no  resemblance  to  the.  underlying  rocks  and  generally 
differ  markedly  from  them.  It  is  evident  that  transitional 
forms  may  exist,  partly  sedentary  and  partly  transported,  and 
many  soils  and  subsoils  are  thus  of  mixed  origin. 

The  character  of  both  sedentary  and  transported  deposits  is 
to  a  large  extent  controlled  by  climate,  but  it  is  necessary  to 
remember  that  in  some  instances  the  dominant  factor  is  the 
climate  of  a  period  which  has  long  passed  away,  as  in  the  well- 
known  instance  of  the  glacial  deposits  of  Europe  and  North 
America.  The  special  influence  of  climate  on  rock- weathering  has 
already  been  dealt  with  (see  Chapter  n),  and  this  is  evidently  of 
fundamental  importance  also  in  connexion  with  the  sedentary 
deposits  as  above  defined.  Many  of  them  are  in  fact  merely 
masses  of  weathered  rock,  that  have  suffered  disintegration,  but 
have  not  been  removed  by  the  agents  of  transport. 

Sedentary  deposits.  These  are  again  subdivided  into  two 
main  groups: 

(a)     Residual  deposits, 
(6)     Cumulose  deposits. 

The  term  residual  signifies  material  that  has  been  left 
behind  during  the  operation  of  the  ordinary  geological  agents 
of  weathering  and  transport,  and  from  this  statement  the 
general  character  of  the  material  is  almost  self-evident. 
Cumulose  deposits  on  the  other  hand  are  mainly  formed  by  the 
growth  and  decay  of  plants  and  to  a  much  less  extent  of  animals. 

(a)  Residual  deposits.  Since  the  majority  of  rocks  are  not 
homogeneous,  but  are  made  up  of  materials  of  varying  chemical 
and  physical  properties,  the  effects  of  weathering  on  these 
different  constituents  are  also  variable.  Some  minerals  are 
more  readily  soluble  than  others,  and  are  more  easily  attacked 
by  acids  or  other  agents.  Hence  there  is  always  a  tendency  for 
the  more  stable  portions  of  the  rock  to  remain  behind,  while 
the  less  stable  are  removed.  A  simple  example  is  afforded  by 


v]  SUPEKFICIAL  DEPOSITS  113 

certain  rocks  composed  of  grains  of  quartz  and  other  resistant 
minerals  embedded  in  a  cement  of  calcite  (calcareous  sandstone). 
The  effect  of  weathering  on  such  a  rock  is  to  remove  the  more 
soluble  and  softer  cement,  leaving  a  residue  of  sand.  Precisely 
similar  are  the  results  of  weathering  of  sandstones  with  a  cement 
of  iron  oxide ;  this  is  easily  dissolved  by  water  containing  organic 
acids;  hence  sandstones  in  regions  where  there  is  little  me- 
chanical removal  of  solid  material  are  covered  by  a  thick  layer  of 
residual  sand.  Again,  conglomerates  consisting  of  hard  pebbles 
with  a  softer  or  more  soluble  matrix  yield  gravels.  Such 
residual  gravels  formed  by  the  weathering  of  conglomerates  are 
remarkably  well  developed  on  the  Witwatersrand  and  in  other 
parts  of  the  southern  Transvaal,  where  owing  to  the  absence  of 
transport,  weathering  may  extend  to  a  depth  of  several  hundred 
feet1.  Of  very  similar  origin  are  the  plateau  gravels  and 
cannon-shot  gravels  of  Cambridgeshire  and  Norfolk,  formed  from 
glacial  deposits,  especially  boulder-clay,  by  the  removal  of  the 
finer  clayey  material,  leaving  the  larger  constituents  behind  as 
a  residue.  These  consist  for  the  most  part  of  flints,  although 
far-travelled  rocks  are  present  in  some  quantity.  One  of  the 
most  remarkable  examples  of  this  process  is  afforded  by  the 
"Sarsen  stones"  of  certain  parts  of  the  Downs  of  the  south  of 
England,  especially  in  Wiltshire.  Stonehenge  and  other  pre- 
historic monuments  are  largely  built  of  them.  Sarsen  stones 
are  blocks  of  a  kind  of  quartzite  of  lower  Tertiary  (Eocene)  age, 
that  have  been  weathered  out  in  situ,  the  rest  of  the  original 
Tertiary  stratum  having  disappeared.  They  are  closely 
related  to  the  well-known  "Hertfordshire  Pudding  Stone,"  a 
conglomerate  of  flint  pebbles  with  a  cement  of  crystalline 
quartz. 

The  Chalk  of  southern  and  eastern  England  is  frequently 
covered  by  a  layer  of  flint- gravel,  or  by  a  mixture  of  flints  and 
clay.  In  some  instances  the  flint  gravels  are  to  be  regarded  as 
simply  the  insoluble  residue  of  the  Chalk,  the  calcium  carbonate 
having  been  removed  in  solution.  In  other  cases,  however,  the 
gravels  are  of  fluviatile  or  glacial  origin,  coming  therefore 

1  Hatch,  "Conglomerates  of  the  Witwatersrand,"  in  Types  of  Ore  Deposits, 
1911,  p.  202. 

B.  A.  G.  8 


114  SUPERFICIAL  DEPOSITS  [OH. 

within  the  category  of  transported  deposits.  The  origin  of  the 
deposit  known  as  the  clay-with-flints  of  southern  England  has 
given  rise  to  considerable  controversy1. 

The  clay-with-flints  covers  a  considerable  area  in  the  south- 
eastern counties,  and  is  of  special  interest  in  that  it  forms  a 
considerable  part  of  the  subsoil  of  the  famous  experimental 
farm  at  Rothamsted  in  Hertfordshire.  It  consists  of  a  some- 
what variable  mixture  of  flints  and  red  clay  or  loam,  the  latter 
being  generally  somewhat  stiff  and  pasty  in  character.  The 
thickness  is  far  from  being  constant  and  the  clay  sometimes 
descends  into  deep  pipes  in  the  underlying  Chalk.  It  was 
formerly  believed  that  the  whole  accumulation,  including  the 
clay,  was  simply  the  insoluble  residue  of  the  Chalk,  since  many 
limestones  are  known  to  yield  a  red  clay  on  weathering  (see 
p.  115).  But  it  has  been  shown  that  the  clay  is  present 
in  too  great  quantity  in  proportion  to  the  flints  to  have 
been  formed  solely  in  this  way,  and  it  is  now  believed  that 
much  of  the  material  is  the  residue  of  formerly  existing  Eocene 
strata,  now  mainly  removed.  It  is  even  suggested  that,  north 
of  the  Thames,  the  clay-with-flints  may  be  partly  or  mainly  of 
glacial  origin2.  However,  south  of  the  Thames,  whether  wholly 
Cretaceous  or  partly  also  Eocene,  it  is  at  any  rate  a  true  residual 
deposit. 

The  greatest  developments  of  residual  weathering  products 
are  found  in  tropical  regions,  where  chemical  and  bacterial 
processes  are  specially  active,  and  thick  vegetation  prevents 
transport  of  the  disintegrated  material.  Under  these  conditions 
weathering  may  extend  downwards  for  hundreds  of  feet  and 
there  is  a  perfectly  gradual  transition  upwards  from  unaltered 
rock  to  soil.  This  helps  to  account  for  the  extreme  fertility  of 
some  tropical  districts.  Under  certain  well-defined  climatic 
conditions,  however,  disintegration  is  not  by  any  means  uniform 
throughout  the  mass  of  the  rock ;  on  the  contrary,  in  countries 
subjected  to  great  daily  variations  of  temperature,  rocks, 
especially  igneous  rocks,  tend  to  break  up  into  masses  along 
their  dominant  joints.  The  edges  and  corners  of  the  blocks  are 

1  Jukes-Browne,  Quart.  Journ.  Geol.  Soc.  vol.  LXII.  1906,  p,  132. 

2  Sherlock  and  Noble,   Quart.  Journ.  Geol.  Soc.  vol.  LXVIII.  1912,  p.  202. 


v]  SUPERFICIAL  DEPOSITS  115 

rounded  off  by  weathering  and  the  ground  is  covered  with  piles 
of  blocks  of  varying  size  and  shape.  For  these  the  name  of 
boulders  of  disintegration  has  been  suggested.  Well-known 
examples  are  the  granite  kopjes  of  Mashonaland  (e.g.  the 
Matoppo  Hills)  and  the  dolerite  kopjes  of  the  Karroo  in  South 
Africa.  Very  similar  in  their  origin  are  the  masses  of  rock 
debris  that  cover  the  outcrops  of  granite  and  other  igneous  rocks 
in  many  parts  of  the  world.  In  our  own  country  masses  of  rock 
fragments  of  this  nature  on  a  large  scale  are  mainly  formed  at 
high  elevations  or  on  very  steep  slopes  and  are  therefore  not 
of  much  agricultural  importance.  In  some  districts  however, 
where  climatic  conditions  are  suitable,  the  outcrops  of  large 
masses  of  granite  are  so  thickly  covered  with  an  accumulation 
of  partly  weathered  blocks,  that  even  at  low  elevations  and  on 
comparatively  level  surfaces  the  ground  is  rendered  perfectly 
useless.  Such  for  example  is  the  case  in  certain  parts  of 
Ireland,  especially  in  Donegal. 

Perhaps  the  simplest  of  all  residual  deposits  are  those  formed 
by  solution  of  part  of  the  rock.  This  occurs  most  commonly 
in  limestone  and  dolomite-rock.  Calcium  and  magnesium 
carbonates  are  soluble  in  water  with  comparative  ease  and  the 
impurities  present  in  the  rock  are  left  behind  on  the  surface. 
These  are  mainly  argillaceous  or  ferruginous  in  character  and 
often  give  rise  to  red  or  brown  soils.  Even  the  Chalk  is 
sometimes  covered  by  a  brown  loamy  soil  of  this  nature,  as  also 
are  the  Carboniferous  Limestone  and,  even  more  markedly,  the 
Jurassic  limestones  of  the  south-western  and  midland  counties. 
These  limestones  are  often  distinctly  ferruginous  and  the  iron 
becomes  concentrated  in  the  less  soluble  residue.  Of  similar 
origin  is  the  residual  deposit  known  as  terra  rossa,  which  covers 
great  areas  in  southern  and  south-eastern  Europe,  being 
characteristically  developed  in  what  is  known  as  the  Karst 
area,  to  the  east  of  the  Adriatic.  The  accumulation  of  terra 
rossa  is  favoured  by  a  rather  deficient  rainfall,  so  that  the 
material  is  not  removed  by  mechanical  transport. 

In  tropical  and  semi-tropical  regions  vast  areas  are  covered 
by  the  deposit  known  as  laterite.  This  varies  greatly  in 
composition,  but  may  be  described  in  general  terms  as  a  mixture 

8—2 


116  SUPERFICIAL  DEPOSITS  [CH. 

in  differing  proportions  of  hydroxides  of  iron  and  alumina, 
with,  in  addition,  more  or  less  manganese  and  titanium  oxides. 
The  latter  are  usually  not  of  much  importance,  but  some  Indian 
laterites  are  of  value  as  ores  of  manganese.  Laterite  varies  in 
composition  from  nearly  pure  iron  hydroxide  to  nearly  pure 
aluminium  hydroxide  (bauxite). 

The  origin  of  laterite  has  given  rise  to  much  controversy. 
It  may  however  be  ascribed  to  a  peculiar  type  of  weathering 
of  rocks  rich  in  alumina  and  iron  under  certain  special  climatic 
conditions.  The  most  characteristic  feature  of  this  type  of 
weathering  is  removal  of  silica  in  solution,  probably  owing  to 
hydrolysis  of  silicates.  Although  laterite  is  most  common  in 
tropical  and  subtropical  countries  it  is  not  exclusively  confined 
to  them,  but  it  appears  that  strongly  contrasted  wet  and  dry 
seasons  are  essential.  Some  authors  attribute  great  importance 
to  deposition  of  dissolved  material  from  ascending  solutions, 
the  ascent  being  conditioned  by  rapid  evaporation  during  dry, 
hot  seasons.  Others  regard  the  process  as  mainly  one  of 
decomposition  in  situ,  while  it  has  even  been  suggested  that  the 
decomposition  may  be  due  to  bacteria,  possibly  allied  to  the 
well-known  nitrifying  bacteria  of  soils.  This  is  however  mainly 
speculative1. 

Laterite  is  most  highly  developed  on  basic  igneous  rocks, 
such  as  the  Deccan  basalts  of  India,  but  it  has  also  been  formed 
from  schist,  gneiss,  slate,  sandstone  and  granite.  Indian 
geologists  recognize  two  types  of  laterite,  high-level  and  low- 
level,  the  former  being  a  true  residual  deposit,  while  the  latter 
has  undergone  transport,  occurring  mostly  as  a  cementing 
material  in  sands  and  gravels. 

Normal  laterite  is  a  distinctly  red  or  brown  substance,  often 
with  ferruginous  nodules  and  concretions.  It  is  fairly  soft 
when  freshly  dug,  so  that  it  can  be  made  into  bricks,  but  it 
becomes  very  hard  on  exposure  to  the  air.  Besides  its  very 
widespread  occurrence  in  India  it  is  also  known  in  the  Malay 
Peninsula,  Java,  Sumatra,  South  America  and  many  parts  of 
Africa;  in  fact  it  seems  to  be  one  of  the  commonest  residual 

1  For  a  summary  of  recent  views  on  the  origin  of  laterite  see  Hatch  and 
Eastall,  The  Petrology  of  the  Sedimentary  Rocks,  London,  1913,  pp.  326-331. 


SUPERFICIAL  DEPOSITS 


117 


products  of  igneous  rocks  under  suitable  conditions  of  climate, 
and  according  to  Ramann  it  is  the  characteristic  superficial 
deposit  of  the  tropics  (see  also  p.  57). 

In  arid  regions  there  is  always  a  considerable  upward 
movement  of  water  by  capillarity  from  the  deeper  layers  of 
the  subsoil  and  from  the  underlying  rocks,  to  replace  that  lost 
by  evaporation  at  the  surface.  This  water  contains  a  good  deal 
of  matter  in  solution,  especially  silica  or  calcium  carbonate. 
These  substances  tend  to  deposit  themselves  in  the  interstitial 
spaces  of  the  surface  deposits,  gravels,  sands  or  soils  as  the 
case  may  be,  thus  often  producing  a  hard  layer,  either  actually 
on  the  surface  or  just  below  it.  Where  this  occurs  it  offers  an 
insuperable  obstacle  to  cultivation. 

The  siliceous  deposits  show  every  gradation  from  a  loosely 
cemented  gravel- conglomerate  to  a  fine-grained  and  intensely 


Fig.  31.     Surface  quartzite  (marked  with  circles)  in  a  bed  of  sand, 
overlain  by  soil  (dotted). 

hard  quartzite  of  a  white,  grey  or  yellowish  colour;  some 
specimens  are  even  bright  green.  Sometimes  the  sand-grains 
are  readily  distinguishable;  sometimes  the  cementing  material 
is  deposited  in  crystalline  continuity  with  the  grains,  so  that 
the  outlines  of  the  latter  are  completely  invisible.  The  rock 
often  has  a  conchoidal  or  splintery  fracture  and  is  sometimes 
almost  like  a  chert  in  texture.  The  hard  bed,  which  may  be 
as  much  as  10  feet  thick,  passes  downwards  by  imperceptible 
gradations  into  loose  sand  or  clay.  The  hardened  portions  also 
frequently  form  large  flat  masses  lying  isolated  in  a  sandy  or 
argillaceous  matrix  or  even  in  laterite.  Surface  quartzites  are 
very  common  in  the  south-western  part  of  Cape  Colony  and  also 
in  parts  of  Bechuanaland1. 

1  Rogers  and  Du  Toit,  Geology  of  Cape  Colony,  2nd  edition,  1909,  pp.  378-385. 


118  SUPERFICIAL  DEPOSITS  [CH. 

Large  areas  in  the  central  and  northern  portions  of  the 
Union  of  South  Africa  are  covered  by  a  calcareous  deposit 
(surface  limestone)  whose  origin  is  apparently  somewhat  similar 
to  that  of  the  quartzites  just  described.  The  rocks  of  the 
Karroo  series  contain  a  good  deal  of  lime,  and  the  calcium 
carbonate  is  formed  by  the  weathering  of  these  rocks.  In  many 
places  the  calcareous  material  attains  a  thickness  of  12  feet1. 
It  often  forms  a  kind  of  pan  immediately  below  the  soil,  and 
it  is  seen  also  in  northern  Cape  Colony  below  a  layer  of  red  sand. 
In  such  cases  it  is  often  nodular  in  structure  and  may  be 
compared  with  the  kankar  of  India. 

(b)  Cumulose  deposits.  The  second  class  of  the  sedentary 
deposits  comprises  all  those  accumulations  that  have  been 
formed  by  the  growth  in  place  of  vegetation,  aided  in  some 
cases  by  the  entangling  of  solid  material  among  the  stems  and 
roots  of  the  plants.  Material  of  animal  origin  plays  a  small 
part  compared  to  vegetable  matter,  although  it  may  be  locally 
of  some  significance.  By  far  the  most  important  of  the  cumulose 
deposits  are  peat-bogs  and  swamps  of  various  kinds.  They  are 
often  closely  associated  with  alluvium  and  certain  types  might 
be  assigned  with  almost  equal  correctness  to  either  group. 

The  term  peat  is  of  somewhat  wide  and  vague  application, 
being  generally  applied  to  all  those  soils  and  surface  deposits 
that  are  abnormally  rich  in  humus  and  fibrous  vegetable  matter 
in  a  more  or  less  advanced  state  of  decomposition.  The 
formation  of  peat  is  specially  characteristic  of  cold  temperate 
and  arctic  climates,  and  does  not  take  place  to  any  extent  in 
the  tropics.  It  is  certainly  brought  about  by  a  special  type  of 
bacterial  action  under  definite  conditions,  the  most  important 
of  these  being  saturation  with  water  and  a  limited  supply  of 
air.  Peat  formation  may  therefore  be  described  as  limited 
oxidation,  leading  to  the  formation  of  compounds  of  the  hydro- 
carbon group. 

In  Britain  two  types  of  peat  are  recognized,  namely,  hill  peat 
and  fen  peat.  Hill  peat  is  chiefly  found  at  considerable  heights 
above  sea-level  in  mountain  and  moorland  regions ;  only  in  the 

1  Hatch  and  Corstorphine,  The  Geology  of  South  Africa,  2nd  edition,  1909, 
p.  329. 


v]  SUPERFICIAL  DEPOSITS  119 

moist  climate  of  western  Scotland  and  of  Ireland  does  it  descend 
to  sea-level.  Hill  peat  consists  mainly  of  mosses,  especially 
Sphagnum  moss,  mixed  with  cotton-grass,  rushes  and  roots  of 
heather.  The.  upper  layers  are  generally  somewhat  brown  and 
fibrous,  becoming  more  compact  and  darker  in  colour  down- 
wards. The  lowest  layers  are  often  quite  black  and 
homogeneous,  approaching  lignite  in  character.  Roots  and 
trunks  of  trees,  often  quite  black,  are  commonly  found  buried 
in  peat  bogs,  indicating  the  former  existence  of  forests  in  what 
are  now  treeless  regions.  Of  late  years  the  peat  bogs  of  Scot- 
land and  northern  England  have  been  made  the  subject  of 
careful  investigation  from  the  botanical  point  of  view.  It  has 
been  shown  that  the  growth  is  extraordinarily  slow,  the  lower 
layers  of  some  of  the  Scotch  peat-mosses  dating  back  to  the 
later  stages  of  the  glacial  period,  and  containing  remains  of 
truly  arctic  plants.  Interstratified  with  these  are  so-called 
"forest  beds"  indicating  a  climate  somewhat  warmer  than  at 
present,  when  trees  grew  at  much  higher  elevations.  Although 
the  conditions  in  Scotland  now  appear  eminently  favourable 
to  the  formation  of  peat,  it  has  been  shown  that  its  growth  is 
practically  at  a  standstill,  and,  indeed,  in  places  it  is  even  being 
destroyed  by  denudation1. 

Fen  peat  on  the  other  hand  is  found  at  low  levels  in  flat 
marshy  regions,  such  as  the  Fenland  of  eastern  England,  the 
"moors"  of  Somerset  (Sedgmoor  and  others)  and  certain  parts 
of  central  Ireland.  It  is  a  substance  of  a  much  more  muddy 
character  than  hill  peat,  consisting  mainly  of  the  remains  of 
water  plants,  such  as  rushes,  sedges  and  grasses,  with  sometimes 
abundance  of  moss  (especially  Hypnum).  Interstratified  with 
the  peat  of  the  Fenland  are  layers  rich  in  remains  of  forest  trees. 
Near  Ely  five  such  forest-beds  have  been  found,  one  above  the 
other,  each  characterized  by  a  different  species  of  tree,  and  a 
similar  succession  has  been  observed  in  the  peat  bogs  of  Den- 
mark. The  occurrence  of  forest  beds  in  the  fens  can  be  explained 
in  two  ways ;  either  by  the  prevalence  for  a  time  of  a  somewhat 
drier  climate,  or  by  a  slight  elevation  of  the  land,  leading  to 

1  Lewis,  "The  Sequence  of  Plant  Remains  in  the  British  Peat  Mosses," 
Science  Progress,  vol.  n.  1907,  p.  307. 


120  SUPEEFICIAL  DEPOSITS  [CH. 

more  effective  natural  drainage.  Since  the  forest  beds  of 
Scotland  are  known  to  be  due  to  climatic  changes,  those  of  the 
Fenland  may  probably  be  attributed  to  the  same  cause. 

The  greatest  development  of  peat  is  to  be  found  in  the 
frozen  regions  of  the  Arctic  zone,  in  northern  Russia,  Siberia, 
Alaska  and  northern  Canada,  constituting  the  Tundra.  Here 
the  annual  average  temperature  is  very  low,  the  ground 
remaining  permanently  frozen  below  a  depth  of  3  or  4  feet,  the 
limit  to  which  the  warmth  of  summer  can  penetrate.  These 
conditions  are  specially  favourable  to  the  peculiar  type  of 
limited  decomposition  that  results  in  peat.  The  dominant 
plants  are  mosses,  especially  Sphagnum,  together  with  dwarf 
birch  and  willow,  Empetrum  and  other  flowering  plants.  The 
whole  surface  of  the  ground  is  composed  of  peat,  usually  in 
undulating,  hillocky  masses,  with  only  occasional  outcrops  of 
bare  rock.  The  Tundra  may  be  considered  as  absolutely  value- 
less agriculturally,  being  quite  incapable  of  cultivation,  or  of 
maintaining  any  animals  except  the  reindeer  and  the  musk-ox1. 

Peat  accumulations  of  various  types  cover  very  large  areas 
in  Germany,  Holland,  Denmark  and  the  Baltic  provinces  of 
Russia.  They  are  of  very  great  commercial  importance  and 
are  now  exploited  on  a  large  scale  as  fuel,  for  moss-litter  and 
for  many  other  economic  purposes.  The  literature  of  the 
subject  is  enormous  and  many  varieties  have  been  recognized, 
based  mainly  on  differences  in  the  botanical  composition  of  the 
peat.  In  Germany,  as  in  Britain,  there  appear  to  be  two  main 
types,which  may  for  convenience  be  called  dry  peat  (Trockentorf ) 
and  swamp  peat.  The  term  dry  here  is  however  only  of  relative 
value.  The  dry  varieties  include  such  forms  as  birch  peat, 
pine  peat,  heather  peat,  bilberry  peat  and  so  forth,  while  the 
swamp  varieties  are  named  arundinetum,  cyperacetum, 
hypnetum,  according  to  the  dominant  species  of  grass,  sedges 
and  mosses.  The  common  sphagnum  peat,  which  is  mainly 
used  for  the  preparation  of  moss-litter,  appears  to  be  a  somewhat 
intermediate  form2. 

1  Ramann,   Bodenkunde,  3rd  edition,   Berlin.   1911,  p.  578.     Glinka,   Die 
Typen  der  Bodenbildung,  Berlin,  1914,  pp.  236-242. 

2  Ramann,  Bodenkunde  3rd  edition,  Berlin,  1911,  pp.  171-186  and  231-238. 


v]  SUPEKFICIAL  DEPOSITS  121 

As  before  stated,  the  essential  condition  for  peat-formation 
is  slow  decomposition  of  vegetable  matter  with  a  limited 
supply  of  oxygen  at  a  comparatively  low  temperature.  This 
can  only  be  attained  when  the  whole  mass  is  saturated  with 
stagnant  water.  Free  circulation  of  water  hinders  the  process, 
leading  to  free  oxidation  and  probably  to  a  different  kind  of 
bacterial  action.  Hence  peat-formation  always  indicates 
deficient  drainage  and  excess  of  ground  water,  which  may  be 
frozen,  as  in  the  Tundra.  A  heavy  rainfall  is  of  course  a  con- 
tributing factor  of  importance,  but  this  alone  will  not  give  rise 
to  growth  of  peat,  as  shown  by  its  cessation  in  western  Scotland, 
where  the  climatic  conditions  appear  to  be  ideal.  In  all 
probability  the  temperature  here  is  somewhat  too  high,  and  it 
is  clear  that  a  subarctic  climate  is  the  most  favourable1. 

The  chemical  composition  of  peat  is  as  a  rule  remarkably 
constant,  as  shown  in  the  following  table;  the  most  variable 
constituent  is  the  mineral  matter,  or  ash]  even  when  this  is 
present  in  considerable  quantity  it  is  often  deficient  in  potash 
and  lime,  and  hence  contains  little  available  plant  food.  On 
the  other  hand  iron  is  abundant,  and  sometimes  gives  rise  to 
deposits  of  iron-ore  in  connexion  with  peat  bogs.  The  forma- 
tion of  pans  is  also  a  well-known  phenomenon  (see  p.  140). 


Thesy,  Forest  Peat, 

France  peat  Sweden 

Carbon         ...       50-67  51-47  51-38 

Hydrogen    ...         5-76  5-96  6-49 

Oxygen        ...       34-951  (35-43 

Nitrogen      ...          1-92]  (  1-68 

Ash  .,                       6-70  9-67  5-02 


On  the  whole  lowland  peat  is  much  richer  in  ash  than  hill 
peat ;  according  to  Ramann2  the  amount  in  the  former  averages 
10  per  cent.,  in  the  latter  2  per  cent.  The  ash  in  each  case  is 
constituted  as  follows: 

Nitrogen  Lime  Phosphoric  acid          Potash 

Hill  peat        0-8%  0-25%  0-05%  0-03% 

Fen  peat        2-5  4-0  0-25  0-10 

1  Arber,  The  Natural  History  of  Coal,  Cambridge,   1911,  pp.  54-64. 

2  Bodenkunde,  Berlin,  1911,  p.  234. 


122  SUPEKFICIAL  DEPOSITS  [CH. 

The  differences  here  are  very  notable,  and  are  quite  sufficient 
to  account  for  the  comparative  fertility  of  the  peaty  Fenland 
under  cultivation.  The  hill  peat  is  strangely  deficient  in  all 
mineral  plant  foods.  These  figures  emphasize  strongly  the 
need  for  potash  manures  on  peaty  soils  in  general. 

With  regard  to  true  swamp  deposits  little  information  i& 
available.  Many  swamps  are  the  silted-up  beds  of  lakes  and 
rivers;  hence  they  come  more  correctly  under  the  heading 
of  alluvium,  since  much  of  the  material  has  been  transported. 
This  also  applies  to  a  large  extent  to  the  mangrove  swamps  of 
tropical  coasts  and  still  more  forcibly  to  river  deltas,  where 
swamps  are  characteristic.  The  so-called  swamp  soils  of  the 
United  States,  allowing  for  differences  of  vegetation,  appear 
from  the  published  descriptions  to  be  very  similar  to  the  lowland 
peat  or  fen  peat  of  Europe.  In  the  silting  up  of  shallow  lakes 
and  pools  with  no  outlet,  or  with  merely  small  streams  running 
through  them,  microscopic  organisms,  both  vegetable  and 
animal,  seem  to  play  a  considerable  part  in  the  earlier  stages, 
often  forming  a  floating  crust  that  gradually  grows  thicker  and 
eventually  forms  a  soil  suitable  for  the  growth  of  higher  plants. 
In  all  such  instances  the  final  product  is  of  a  peaty  nature, 
with  a  very  high  proportion  of  humus,  and  a  small  amount  of 
inorganic  material  mainly  derived  from  the  ash  or  mineral 
constituents  of  the  decayed  plants. 

Transported  deposits.  Under  normal  climatic  conditions 
and  more  especially  in  temperate  and  cold  regions,  by  far  the 
greater  part  of  the  superficial  deposits  have  undergone  more  or 
less  transport;  frequently  the  whole  mass  has  been  brought 
from  a  distance  and  may  reach  a  great  thickness.  The  geological 
agents  here  specially  involved  are  running  water,  ice  and  wind. 
Each  of  these  gives  rise  to  a  special  type  of  deposit,  needing 
separate  consideration.  Gravity  is  of  minor  importance  and 
local  in  its  action,  depending  solely  on  the  configuration  of  the 
ground. 

Transported  deposits  may  be  classified  on  the  basis  of  the 
geological  agent  chiefly  concerned  in  their  formation,  as  follows1 : 

1  Merrill,  Rocks,  Rock-weathering  and  Soils,  1897,  p.  300. 


v]  SUPEKFICIAL  DEPOSITS  123 

Colluvial  ...  Screes  and  cliff  debris. 

Alluvial  ...  River  and  swamp  deposits,  loess  (in  part). 

Glacial  ...  Moraines,  drumlins,  boulder  clay,  gravels. 

Aeolian  ...  Wind-blown  sand,  loess  (in  part). 

The  only  term  requiring  explanation  is  colluvial.  This 
is  derived  from  the  Latin  colluvies,  a  heap;  all  the  other 
terms  are  in  common  use,  and  their  significance  is  obvious. 

Colluvial  deposits.  These  are  of  little  agricultural  import- 
ance, being  mainly  found  in  uncultivated  areas.  In  temperate 
regions  they  are  mainly  confined  to  mountainous  situations, 
forming  screes  at  the  base  of  precipices  and  steep  slopes  generally. 


Fig.  32.  Formation  of  a  scree  at  the  foot  of  a  precipice,  consisting  of  angular 
fragments  of  rock,  shattered  by  frost-action  or  by  changes  of  temperature. 
Such  a  deposit,  when  consolidated,  forms  a  breccia. 

In  the  case  of  screes  the  transport  is  mainly  due  to  gravity, 
although  in  some  instances  running  water  also  plays  a  part. 
The  greatest  development  of  screes  is  undoubtedly  in  arid 
regions,  such  as  Baluchistan  and  the  Sinai  peninsula.  Here  the 
main  agent  of  disintegration  is  change  of  temperature.  In 
colder  climates  frost  acts  in  a  similar  way  (e.g.  the  well-known 
screes  of  the  Lake  District). 


124  SUPERFICIAL  DEPOSITS  [CH. 

In  certain  parts  of  the  south  of  England,  beyond  the  southern 
limit  of  the  Pleistocene  ice-sheet,  considerable  areas  are  covered 
by  accumulations  which  may  conveniently  be  placed  in  this 
category.  Such  are  the  Coombe  Rock  or  Elephant-bed  of 
the  South  Downs  and  the  Head  of  Cornwall.  The  Coombe 
Rock  is  found  in  the  dry  valleys  or  coombes  of  the  Downs,  and 
consists  of  a  thick  mass  of  angular  flints,  with  more  or  less 
loamy  matrix.  It  contains  in  abundance  broken  and  decayed 
teeth  of  horse  and  elephant.  When  traced  to  the  lower  ground 
the  larger  elements  become  relatively  less  abundant,  and  the 
Coombe  Rock  appears  to  pass  laterally  into  the  brick-earths  of 
the  coastal  plain  of  Sussex.  It  is  supposed  that  this  deposit 
and  the  Head  of  Cornwall  were  formed  during  the  glacial  period, 
when  the  ground  was  frozen  and  the  rainfall  could  not  sink  in, 
as  it  now  does,  but  flowed  over  the  surface  in  torrential  streams1. 

Many  water-borne  deposits  might  also  be  assigned  to  this 
class,  as  for  example  the  piles  of  blocks  and  debris  so  often  seen 
along  the  courses  of  mountain  streams,  especially  at  the  points 
where  they  reach  the  lowlands  or  where  a  tributary  joins  a 
main  valley.  These  are  often  called  alluvial  cones,  but  the 
name  is  scarcely  appropriate  in  this  sense.  Such  deposits  may, 
in  course  of  time,  become  covered  by  finer  material  and  by  soil 
and  they  sometimes  give  rise  to  good  pasture  in  mountainous 
regions. 

Alluvial  deposits.  This  is  a  very  comprehensive  term  and 
includes  a  great  variety  of  superficial  deposits,  all  of  which  are 
due  directly  or  indirectly  to  running  water.  They  cover  vast 
areas,  especially  in  low-lying  lands  and  in  river  valleys,  and  they 
constitute  the  subsoil  of  some  of  the  most  fertile  regions  of  the 
world.  As  examples  of  alluvium  on  a  large  scale  special  mention 
may  be  made  of  the  Mississippi  valley,  the  deltas  of  the  Ganges 
and  Bramaputra,  of  the  Nile  and  other  African  rivers,  and  the 
great  plains  of  China.  Delta  deposits  may  perhaps  be  regarded 
as  the  typical  alluvium,  but  the  flood  plains  of  large  rivers  are 
also  very  important,  while  along  the  courses  of  nearly  all  rivers, 
at  any  rate  in  their  lower  reaches,  more  or  less  alluvium  is 

1  Reid,  Quart.  Journ.  Geol.  Soc.  vol.  XLIII.  1887,  p.  364.  Elsden,  "Geology 
in  the  Field."  Geol.  Ass.  Jubilee  Volume,  1910,  p.  275. 


v]  SUPEEFICIAL  DEPOSITS  125 

found.  Fertile  alluvial  plains  are  also  formed  by  the  silting 
up  of  lakes. 

Alluvial  deposits  vary  enormously  in  character,  ranging  from 
piles  of  immense  boulders  along  the  courses  of  mountain 
streams,  through  all  grades  of  size  down  to  the  finest  silt  and 
warp  of  tidal  rivers.  They  may  therefore  be  referred  to  all  the 
normal  categories  of  sediment ;  boulders,  gravel,  sand  and  mud. 
A  distinctive  feature  of  most  alluvium  is  a  high  proportion  of 
organic  matter,  and  many  alluvial  deposits  might  almost 
equally  well  be  classified  as  peat  and  swamp  soils. 

The  coarse-textured  boulder  deposits  of  torrential  streams 
in  hilly  regions  are  of  little  or  no  agricultural  importance  and 
may  be  disregarded.  River  gravels  of  moderately  coarse  grain, 
with  abundant  pebbles  of  2  or  3  inches  in  diameter,  cover  very 
considerable  areas  in  comparatively  low  ground,  and  are  of 
some  importance,  as  for  example  along  the  chief  rivers  of 
southern  England,  and  especially  the  Thames.  The  gravels  of 
the  Thames  are  clearly  divided  into  terraces,  at  different  heights 
above  the  present  river  level,  and  the  same  statement  applies  to 
the  gravels  of  the  Great  Ouse,  the  Cam  and  other  rivers  of  the 
eastern  counties.  The  generally  accepted  explanation  of  these 
terraces  is  that  they  belong  to  periods  when  the  land  stood  at 
different  heights  above  sea-level.  The  oldest  and  highest,  the 
100  foot  terrace  of  the  Thames,  was  formed  by  that  river  when 
the  land  was  generally  about  100  feet  lower  than  at  present. 
When  an  uplift  occurred,  the  equilibrium  of  the  river  was 
disturbed  and  it  was  forced  to  deepen  its  channel,  in  the 
endeavour  to  establish  the  base-line  of  erosion  proper  to  the 
new  conditions.  The  new  valley  was  narrower  than  the  old 
one  and  some  of  the  alluvium  of  the  older  period  was  left  as  a 
terrace  on  the  sides  of  the  valley.  A  repetition  of  this  process 
gave  rise  to  a  still  narrower  valley  and  formed  the  50  foot 
terrace.  From  a  study  of  the  flint  implements  and  other 
human  relics  found  in  the  gravels  it  has  been  found  possible 
to  correlate  the  formation  of  such  gravel  terraces  with  different 
stages  of  culture  (Palaeolithic  man),  and  with  the  occurrence 
of  certain  types  of  extinct  animals,  especially  different  species 
of  elephant,  rhinoceros  and  hippopotamus.  Fig.  33  shows  the 


126  SUPEKFICIAL  DEPOSITS  [CH. 

relation  of  such  gravel  terraces  to  each  other  and  to  the  under- 
lying older  strata1. 


Fig.  33.     Gravel  terraces  along  the  course  of  a  river.     The  figures  show  two 
terraces  and  the  gravels  formed  at  the  present  river  level. 

Besides  the  gravel  terraces  clearly  belonging  to  earlier 
stages  of  existing  rivers,  there  are  also  in  the  eastern  counties 
obvious  river-gravels  formed  by  rivers  which  have  long  ago 
disappeared.  Some  of  the  best  known  and  most  interesting  of 
these  are  in  Cambridgeshire.  The  gravels  of  the  ancient  river 
system  start  in  dry  valleys  in  the  Chalk  near  Newmarket ;  they 
can  be  traced  as  low  ridges  over  the  lower  ground,  crossing 
the  present  Cam  at  right  angles  just  below  Cambridge  and 
running  towards  St  Ives.  Near  March  they  can  be  followed 
laterally  into  marine  gravels  with  shells  of  a  somewhat  arctic 
character,  showing  that  at  the  time  of  their  formation  the  sea 
extended  far  into  what  is  now  the  Fenland.  All  these  gravels 
consist  mainly  of  flints,  with  a  very  small  admixture  of  other 
stones;  indeed  on  all  Chalk  tracts  the  conditions  are  specially 
favourable  to  the  formation  of  river  gravels,  owing  to  the 
abundance  and  durability  of  flints.  The  pebbles  of  the  Thames 
gravels  are  partly  derived  directly  from  the  Chalk,  but  also 
largely  in  an  indirect  manner  from  the  Tertiary  strata.  In 
Norfolk  and  Cambridgeshire  the  gravels  contain  far-travelled 
stones  derived  from  the  glacial  drift  and  originally  transported 
by  the  ice  from  northern  England,  Scotland  and  Norway. 
River  gravels  also  cover  large  areas  in  the  Trent  valley,  especially 
near  Newark,  extending  as  far  east  as  Lincoln.  It  is  believed 
that  the  Trent  once  flowed  through  the  gap  in  the  Jurassic 
escarpment  at  Lincoln,  and  that  its  present  course  by  Gains- 
borough to  the  Humber  is  of  comparatively  recent  date.  The 
presence  of  these  gravel  deposits  helps  to  confirm  this  view. 

1  Salter,  "On  the  Superficial  Deposits  of  Central  and  Parts  of  Southern 
England,"  Proc.  Geol.  Ass.  vol.  xix.  1905,  p.  1. 


v]  SUPEKFICIAL  DEPOSITS  127 

The  majority  of  river  deposits  in  low-lying  and  cultivated 
regions  are  of  finer  grain  than  the  foregoing,  ranging  from 
.sand  through  loam  to  silt,  and  in  estuaries  even  to  mud. 
These  constitute  the  basis  of  some  of  the  most  fertile  of  all 
known  soils,  especially  along  the  lower  courses  of  the  great 
rivers  of  tropical  and  subtropical  countries.  In  temperate  and 
cold  climates  river  alluvium  often  shows  a  peaty  tendency,  but 
in  the  tropics  oxidation  and  bacteria  are  too  active  for  the 
growth  of  peat  to  attain  its  full  development.  The  nature  of 
the  alluvium  brought  down  by  any  given  river  must  naturally 
depend  on  the  character  of  the  rocks  over  which  it  flows,  hence 
infinite  variety  is  possible,  and  since  few  rivers  flow  solely  over 
one  kind  of  rock,  alluvium  is  generally  of  a  very  mixed  nature. 
This  accounts  to  a  large  extent  for  its  general  fertility,  since 
all  the  constituents  of  plant  food  are  likely  to  be  present. 

The  best  examples  of  alluvium  are  to  be  found  in  countries 
that  have  not  been  glaciated  in  geologically  recent  times. 
Within  the  area  overrun  by  the  Pleistocene  ice-sheet  there  is 
always  much  glacial  material  mixed  with  the  alluvium,  and  this 
is  specially  the  case  in  the  British  Isles.  North  of  the  Thames 
it  is  almost  impossible  to  find  alluvial  deposits  free  from  far- 
travelled  ice-borne  material,  and  even  in  the  southern  counties 
it  is  yet  doubtful  to  what  extent  ice  played  a  part  in  the 
formation  of  the  superficial  deposits.  The  great  "alluvial" 
tracts  of  the  Fenland  and  of  the  Humber  estuary  are  not  true 
river  alluvium,  but  consist  mainly  of  marine  silt  and  peat  (see 
pp.  119  and  128). 

The  delta  of  the  Nile  affords  a  particularly  good  example 
of  alluvium,  attaining  a  great  thickness,  as  revealed  by  deep 
borings.  The  deposits  consist  of  irregular  alternations  of  sand 
and  mud,  varying  in  texture  according  to  the  conditions  of 
deposition.  The  larger  sand  grains  are  much  rounded,  showing 
derivation  from  the  desert,  but  as  usual  the  smaller  grains  are 
more  angular.  The  mud  consists  essentially  of  the  same 
materials  as  the  sand  in  a  finer  state  of  division,  and  there  is 
in  addition  a  varying  amount  of  organic  matter.  Owing  to 
the  annual  floods  of  the  Nile,  the  conditions  are  here  specially 
favourable  to  the  accumulation  of  alluvium  over  a  very  wide 


128  SUPERFICIAL  DEPOSITS  [CH. 

area    and    the    soil    produced    from    it    is    also    remarkably 
fertile. 

Along  the  lower  course  of  the  Mississippi  there  is  a  great 
spread  of  river  alluvium  which  yields  a  good  soil.  The 
Mississippi  and  Missouri  are  both  remarkably  muddy  rivers, 
and  it  has  been  estimated  that  they  carry  down  to  the  sea 
annually  more  than  7,000,000,000  cubic  feet  of  solid  matter  in 
suspension.  Consequently  the  delta  is  growing  very  fast  and 
now  extends  a  long  way  into  the  Gulf  of  Mexico.  The  flood 
plain  of  the  Mississippi  is  several  hundred  miles  in  length  and 
many  miles  wide ;  on  either  side  it  is  bounded  by  steep  bluffs 
and  the  area  between  these  is  constantly  flooded  by  the  river, 
thus  receiving  a  fresh  coating  of  finely  divided  sediment 
which  is  added  to  the  soil  and  increases  its  fertility.  Much  of 
the  flood  plain  is  covered  by  thick  forests  and  undergrowth, 
but  there  are  also  considerable  grassy  areas.  Sometimes, 
owing  to  accumulation  of  sediments,  the  streams  naturally 
flow  at  a  higher  level  than  the  general  surface  of  the  ground 
this  feature  being  often  accentuated  by  artificial  banks  or 
"levees."  In  violent  floods  the  rivers  sometimes  change  their 
courses,  leaving  long,  narrow,  partly  dried-up  channels ;  these 
are  called  "ox-bow  lakes."  The  great  alluvial  plains  of  the 
Ganges  and  of  the  rivers  of  China  are  very  similar  in  their 
general  characters  and  are,  for  the  most  part,  extraordinarily 
fertile. 

The  alluvium  of  the  English  Fenland  is  of  a  rather  special 
character.  Although  several  fairly  large  rivers  run  into  the 
Wash,  the  Fenland  is  not  in  the  main  a  delta  deposit  in  the 
ordinary  sense,  since  the  material  has  been  to  a  large  extent 
brought  in  from  the  sea.  The  growth  of  peat  also  plays  a 
considerable  part  (see  p.  119).  The  deposits  of  the  Fenland  are 
of  three  different  types,  namely,  (1)  gravel,  (2)  marine  silt, 
(3)  peat.  The  gravels  come  to  the  surface  furthest  inland, 
forming  a  continuation  of  the  river  gravels  and  probably 
underlying  the  other  deposits  also ;  next  comes  the  peat,  while 
the  silt  is  now  chiefly  seen  at  the  seaward  margin.  However 
the  peat  of  the  landward  region  is  nearly  everywhere  underlain 
by  silt  and  the  different  types  often  alternate  vertically  as  well 


v]  SUPERFICIAL  DEPOSITS  129 

as  horizontally.  The  marine  silt  occurs  in  two  different  types ; 
one  is  a  clay,  the  so-called  buttery  day,  while  the  more  sandy 
type  is  often  called  warp  (compare  the  warp  of  the  Humber, 
to  be  described  later)1.  The  prevalence  of  marine  silt  can 
be  explained  as  follows.  The  coasts  of  north  Lincolnshire  and 
south  Yorkshire  consist  of  soft  strata,  mainly  glacial,  which  are 
being  rapidly  eroded  by  the  sea.  The  set  of  the  tidal  current 
is  here  from  north  to  south,  running  strongly  into  the  opening 
of  the  Wash,  which  formerly  extended  at  least  as  far  as  March 
and  perhaps  further.  Hence  the  fine  silt  carried  by  the  tide 
was  deposited  in  the  slack  water,  and  gradually  filled  up  the 
bay.  When  it  had  reached  a  certain  height  the  growth  of  peat 
began  and  completed  the  process.  The  sandy  warp  forms  a 
fertile  soil  which  of  late  years  has  been  found  admirably  suited 
to  bulb-growing.  This  is  now  quite  an  important  local  industry 
around  Spalding.  Another  crop  peculiar  to  the  Fenland  is 
woad,  which  is  still  grown  to  a  small  extent  near  Wisbech. 

The  waters  of  some  rivers  are  specially  rich  in  suspended 
material  and  these  are  most  liable  to  form  alluvial  deposits. 
The  Humber  is  said  to  be  the  richest  in  silt  of  all  the  British 
rivers,  and  advantage  is  taken  of  this  fact  to  increase  the 
fertility  of  the  adjoining  land  by  artificial  means ;  the  process 
is  known  as  warping,  and  may  be  described  briefly  as  follows. 
Warping  consists  of  letting  in  silt-laden  tidal  water  through 
sluices  and  excavated  drains,  and  allowing  it  to  stand  upon  the 
selected  lands,  the  water  being  gradually  drawn  off  again  with 
the  fall  of  the  tide ;  the  silt  or  warp  is  thus  deposited,  and  the 
process,  when  continued  for  some  time,  results  in  the  formation 
of  a  soil  of  great  fertility.  Some  of  the  warp-lands  of  the 
Humber  are  said  to  be  among  the  richest  soils  of  England2. 

Glacial  deposits.  These  constitute  a  large  group  of  surface 
accumulations  of  very  great  importance  in  the  British  Isles,  in 
North  America  and  indeed  in  all  those  countries  that  came 
within  the  influence  of  the  Pleistocene  glaciation.  Glacial 
deposits  of  recent  age  are  also  found  in  the  higher  mountain 

1  Skertchley.  "The  Geology  of  the  Fenland,"   Mem.  Oeol.   Survey,   1877. 
Rastall.    "The  Geology  of  Cambridgeshire,  Bedfordshire  and  West  Norfolk," 
Geol.  Ass.  Jubilee  Volume,  1910,  p.  177. 

2  Third  and  FinalReport  of  the  Royal  Commission  onCoast  Erosion,  1911, p.  127. 
R.  A.G.  9 


130  SUPERFICIAL  DEPOSITS  [CH. 

ranges  of  almost  all  parts  of  the  world,  but  these  are  of  less 
importance  than  those  of  the  earlier  Pleistocene  period. 
Glacial  deposits  display  very  great  variety  of  composition,  but 
they  can  in  general  be  divided  into  two  main  groups,  namely : 

(1)  Sands  and  gravels, 

(2)  Boulder-clay. 

It  does  not  come  within  the  province  of  this  chapter  to  deal 
with  all  the  characteristics  of  glaciated  regions:  scratched 
surfaces,  roches  moutonnees  and  other  features  of  glacial 
topography  may  be  disregarded,  as  we  are  here  dealing  solely 
with  deposits  that  may  give  rise  to  soil. 

Glacial  deposits  may  be  laid  down  either  by  the  ice  itself, 
as  moraines  and  other  related  structures,  or  by  the  water 
flowing  from  the  ice.  The  latter  originates  many  of  the  masses 
of  sand  and  gravel,  while  boulder- clay  is  apparently  always 
formed  directly  by  the  ice.  The  deposits  of  the  streams  of 
water  flowing  from  the  ice  are  now  commonly  designated 
fluvioglacial,  and  in  recent  times  more  and  more  importance  has 
come  to  be  attributed  to  this  mode  of  origin. 

In  this  connexion  it  must  not  be  forgotten  that  for  many 
years  a  controversy  has  raged  as  to  the  nature  of  the  conditions 
that  gave  rise  to  the  glacial  deposits  of  Pleistocene  age  in 
Britain  and  elsewhere.  On  this  question  geologists  are  ranged 
into  two  schools,  the  one  maintaining  a  general  submergence 
of  the  whole  area  to  a  depth  of  many  hundreds  of  feet  beneath 
the  waters  of  the  sea ;  this  school  considers  that  the  main  work 
of  transport  and  deposition , was  performed  by  floating  ice,  as 
bergs  and  floes.  The  other  school  maintains  that  the  relative 
levels  of  land  and  sea  were  then  much  the  same  as  now,  and  that 
the  whole  area  was  covered  by  an  ice-sheet  like  that  of  Greenland, 
the  motive  power  of  the  ice  being  its  own  weight;  the  vast 
accumulation  of  snow  and  ice  at  the  centre  forced  the  outer 
parts  to  move  for  great  distances,  even  across  the  sea  and  over 
hills  of  considerable  altitude.  Thus  on  this  theory  Scandinavian 
ice  must  have  crossed  the  North  Sea  and  climbed  to  a  height 
of  several  hundred  feet  on  the  coast  of  Yorkshire.  In  spite  of 
the  great  mechanical  difficulties  involved,  the  ice-sheet  theory 


v]  SUPERFICIAL  DEPOSITS  131 

is  now  accepted  by  the  majority  of  geologists.  One  thing  at 
any  rate  is  universally  admitted,  namely,  that  during  the 
glacial  period  there  were  glaciers  of  the  alpine  type  in  the 
mountains  of  Britain,  and  that  these  formed  moraines  and 
other  structures  such  as  can  still  be  seen  in  regions  where 
glaciers  now  exist.  The  main  controversy  is  concerned  with  the 
origin  of  the  glacial  deposits  of  the  midland  plain  and  of  the 
eastern  counties. 

The  essential  feature  for  the  present  purpose  is  that  vast 
areas  of  land  in  Britain  and  elsewhere  are  covered  by  a  thick 
layer  of  drift,  undoubtedly  of  glacial  origin  and  containing 
material  that  has  been  transported  for  great  distances.  This 
drift  varies  much  in  lithological  character  and  in  thickness, 
but  it  is  usually  sufficiently  thick  to  exert  a  preponderating 
influence  in  determining  the  character  of  the  soil. 

The  exact  nature  of  the  glacial  deposits  of  any  region  is 
naturally  determined  by  the  source  of  the  material  composing 
it.  In  most  instances  a  considerable  part  of  the  drift  is  derived 
from  the  underlying  rocks,  but  there  is  always  more  or  less 
admixture  of  material  of  distant  origin.  Thus  the  glacial 
gravels  of  Norfolk  and  Suffolk  consist  mainly  of  flints,  while 
most  of  the  boulder-clay  is  chalky.  In  Cheshire  and  Stafford- 
shire the  drifts  are  largely  made  up  of  Triassic  material,  with 
pebbles  from  the  Bunter  pebble  beds  and  abundant  rock-frag- 
ments from  Scotland  and  the  Lake  District.  The  boulder-clays 
of  the  Midlands  are  made  up  of  debris  from  the  different  Jurassic 
strata,  with  usually  a  good  deal  of  Chalk  from  Lincolnshire. 
Many  other  instances  might  also  be  given,  but  it  will  perhaps 
be  more  satisfactory  to  describe  the  sequence  of  the  drifts  in 
one  or  two  typical  areas  as  examples. 

From  a  study  of  the  drifts  of  Norfolk,  Suffolk  and  Cam- 
bridgeshire it  has  been  found  possible  to  establish  a  definite 
sequence,  as  follows : 

5.     Plateau  gravels. 

4.     Great  Chalky  Boulder-clay. 

3.     Mid-glacial  sands. 

2.     Contorted  drift, 

1.     Cromer  till. 

9—2 


132  SUPERFICIAL  DEPOSITS  [CH. 

Of  these  five  subdivisions  only  the  third  and  fourth  are  of 
much  importance  agriculturally.  The  Cromer  till  and  the  con- 
torted drift  are  hardly  seen  except  in  the  cliffs  of  the  coast.  The 
latter  however  is  supposed  to  form  the  subsoil  of  a  very  fertile 
area  near  Norwich.  The  Cromer  till  is  a  heavy  brown  clay  and 
the  contorted  drift  is  a  light  yellow  loam,  stratified  and  extra- 
ordinarily contorted,  with  blocks  of  Chalk  up  to  one  or  two 
hundred  yards  in  length.  Both  divisions  contain  many  Scotch 
and  Scandinavian  rocks  and  were  clearly  formed  by  ice  coining 
from  the  North  Sea.  Glacial  sands  and  gravels  cover  large 
areas  of  the  three  counties  named ;  they  vary  from  coarse  flint 
gravels  to  fine  sand,  the  latter  often  being  much  wind-blown. 
Owing  to  the  prevalence  of  these  deposits  there  is  much  poor 
sandy  land  in  the  west  of  Norfolk  and  of  Suffolk,  a  large  area 
being  uncultivated  and  under  heather  and  bracken.  This 
kind  of  land  is  of  more  sporting  than  agricultural  value.  The 
glacial  gravels  are  often  difficult  to  distinguish  from  the  river 
gravels  before  described.  Some  of  them  probably  belong  to 
the  mid-glacial  series,  but  part  are  undoubtedly  newer  than  the 
Chalky  Boulder- clay. 

The  Great  Chalky  Boulder-clay  is  the  most  widespread  and 
the  most  important  of  all  the  drift  deposits  of  eastern  and 
central  England.  Although  varying  a  good  deal  in  character 
it  usually  contains  more  or  less  Chalk,  together  with  far- travelled 
boulders,  which  are  sometimes  derived  from  older  glacial 
deposits.  The  nature  of  the  boulder-clay  is  largely  determined 
by  the  kind  of  rock  on  which  it  lies,  since  much  of  the  material 
has  not  been  transported  very  far.  The  argillaceous  portion 
is  mainly  derived  from  the  clays  of  the  Jurassic  and  Cretaceous, 
and  the  colour  and  texture  of  the  boulder- clay  are  determined 
by  this  fact ;  consequently  also  derived  Jurassic  and  Cretaceous 
fossils  are  common.  From  a  study  of  the  distribution  of  the 
different  types  of  boulder- clays  and  their  contents  in  eastern 
and  central  England  it  has  been  found  possible  to  obtain 
much  information  as  to  the  sequence  and  direction  of  the 
ice-streams  of  the  Pleistocene  glaciation.  It  is  clear  that  by 
far  the  greater  part  of  the  material  constituting  the  drifts 
of  eastern  England  is  of  local  origin,  the  foreign  portion, 


v]  SUPERFICIAL  DEPOSITS  133 

though  the  most  noticeable,  being  really  insignificant  in  total 
amount. 

The  plateau  gravels  of  eastern  England  have  been  previously 
mentioned  and  their  manner  of  formation  described  (see  p.  113). 
Such  gravels  of  glacial  or  fluvioglacial  origin  cover  large  areas 
in  Norfolk,  Suffolk  and  Cambridgeshire  and  give  rise  to  land 
which  is  in  places  almost  completely  uncultivated,  being 
mainly  under  bracken  and  heather,  and  sometimes  entirely 
barren.  Similar  spreads  of  high  level  gravels  form  many  of 
the  open  commons  in  the  home  counties  north  of  the  Thames. 
Some  of  these  are  now  becoming  of  value  as  residential  districts, 
especially  in  Buckinghamshire. 

The  glacial  deposits  of  Lincolnshire  and  Yorkshire  resemble 
those  of  East  Anglia  in  general  character,  though  differing  in 
detail.  The  drifts  of  Holderness  have  been  exhaustively 
studied  by  the  Geological  Survey1.  It  is  believed  that  some 
of  the  clay  has  here  been  redeposited  by  wind  (see  also  pp.  136 
and  139). 

The  drifts  of  western  England  differ  a  good  deal  from  those 
of  the  east,  since  they  are  formed  from  very  different  material. 
On  the  plains  of  Lancashire  and  Cheshire  are  found  boulder- 
clays  rich  in  material  derived  from  the  Trias,  especially  from 
the  red  marls  of  the  Keuper.  Rounded  sand  grains  and  pebbles 
from  the  Bunter  are  also  recognizable.  The  far-travelled 
boulders  come  from  various  sources;  from  the  Lake  District 
and  Galloway  and  also  from  the  mountains  of  North  Wales. 
The  latter  are  abundant  as  far  east  as  Birmingham.  Pebbles 
from  the  Bunter  pebble  bed  are  found  scattered  over  the 
surface  in  many  parts  of  the  Midland  counties,  showing  the 
former  existence  of  a  thin  covering  of  drift  which  has  now  mainly 
disappeared  or  become  incorporated  with  the  soil. 

In  almost  all  the  hilly  and  mountainous  regions  of  the 
British  Isles  glacial  deposits  are  very  largely  developed  as 
moraines,  drumlins,  eskers,  gravels  and  boulder-clay.  In  fact 
it  may  be  said  that  in  such  regions  they  constitute  the  com- 
monest subsoil.  They  vary  greatly  in  lithological  character 
and  yield  soils  of  widely  differing  character  according  to  the 
1  "Geology  of  Holderness,"  Mem.  Geol.  Survey,  1885. 


134  SUPERFICIAL  DEPOSITS  [CH. 

kind  of  rock  forming  the  gathering  ground  of  the  glaciers. 
The  soils  are  usually  stony  and  not  very  fertile. 

The  thickness  of  glacial  deposits  varies  enormously,  ranging 
from  a  mere  surface  skin  to  several  hundred  feet.  It  is  evident 
that  a  very  small  thickness  of  drift  is  sufficient  to  determine 
the  character  of  the  soil.  Hence  the  importance  of  the  study 
of  drift  maps.  Sometimes  it  is  shown  by  well-borings  that 
pre- glacial  river  valleys  have  been  filled  by  drift  to  surprising 
depths ;  at  Glemsford,  near  Sudbury  in  Suffolk,  a  boring  passed 
through  470  feet  of  drift,  the  floor  of  the  old  valley  being  nearly 
350  feet  below  present  sea-level.  Near  Newport,  Essex,  340 
feet  of  drift  was  passed  through  and  in  the  north  of  England 
records  of  200  feet  of  drift  are  not  uncommon.  All  this  indicates 
that  in  pre-glacial  times  the  land  stood  much  higher  than  now, 
and  the  area  now  occupied  by  the  North  Sea  was  then  in  all 
probability  an  alluvial  plain  formed  by  the  Rhine  and  its  tribu- 
taries. A  knowledge  of  the  existence  of  such  deep  drift-filled 
channels  is  evidently  of  great  importance  in  questions  of  water- 
supply,  but  unfortunately  they  can  only  be  detected  by  actual 
boring. 

Deposits  formed  by  wind.  Deposits  of  this  class  are  naturally 
most  abundant  in  dry  climates,  and  in  temperate  regions  with 
considerable  rainfall  they  are  of  but  local  and  limited  occurrence. 

In  our  own  country  the  most  conspicuous  wind  deposits  are 
the  sand-hills  of  the  coast.  Owing  to  the  prevalence  of  westerly 
and  south-westerly  winds  they  are  much  more  prevalent  on 
the  west  coast  of  Britain  than  on  the  east;  still  they  are 
developed  to  a  considerable  extent  on  certain  parts  of  the 
eastern  coast,  especially  in  Norfolk  and  in  many  parts  of  Scot- 
land, e.g.  Fife,  Aberdeen,  and  the  shores  of  the  Moray  Firth. 
On  the  west  coast,  wherever  the  shore  is  low  and  not  bounded 
by  cliffs,  sand-hills  are  almost  universal.  The  width  of  the 
belt  of  dunes  is  not  generally  great,  not  often  exceeding  a  few 
hundred  yards.  The  primary  source  of  the  material  is  in 
nearly  all  instances  the  sand  of  the  beach;  this  is  cast  up  by 
the  waves  and  carried  inland  in  large  quantities  by  the  pre- 
vailing winds.  Coastal  sand-hills  are  of  little  or  no  agricultural 
value,  but  are  now  very  commonly  utilized  as  golf-links.  The 


v]  SUPERFICIAL  DEPOSITS  135 

sand-hills  of  the  coast  are  not  always  stationary,  but  often  tend 
to  encroach  on  cultivated  ground,  sometimes  to  a  disastrous 
extent.  This  state  of  things  may  be  seen  in  many  places  in 
England,  especially  in  Cornwall  and  Suffolk,  but  one  of  the  most 
notable  examples  was  the  destruction  of  the  Barony  of  Culbin, 
on  the  southern  shores  of  the  Moray  Firth.  Several  thousand 
acres  of  the  most  fertile  land  in  the  county  of  Elgin  were 
suddenly  overwhelmed  by  sand  in  the  seventeenth  century 
and  have  remained  in  that  condition  ever  since.  In  many 
cases  such  movements  of  sand-hills  have  been  promoted  by 
destruction  of  trees  growing  on  them,  and  conversely  it  has 
been  found  that  such  destructive  movements  can  be  prevented 
by  afforestation,  especially  by  the  planting  of  pine-trees,  as  has 
been  largely  done  in  Norfolk,  on  the  Holkham  estate  and 
elsewhere. 

The  most  extensive  development  of  wind-blown  sands  is 
found  in  the  true  desert  areas  of  the  arid  zone ;  here  however 
agriculture  in  the  true  sense  is  non-existent.  There  is  no  doubt 
that  within  historic  times  the  advance  of  desert  sands  has 
overwhelmed  fertile  and  highly  cultivated  areas,  as  in  parts  of 
the  Nile  valley  and  over  a  large  region  in  south-western  Asia. 

It  is  however  in  South  Africa  and  Australia  that  wind-borne 
deposits  attain  their  greatest  practical  importance.  Where  the 
climate  is  hot  and  dry  changes  of  temperature  and  wind  are  the 
chief  geological  agents,  and  the  main  product  of  their  activity 
is  sand.  In  such  areas  the  vegetation  is  specially  characterized 
by  the  abundance  of  succulent  plants,  capable  of  resisting 
prolonged  drought  and  the  agriculture  therefore  shows  special 
features,  which  cannot  here  be  described  in  detail.  Normal 
arable  farming  can  as  a  rule  only  be  carried  on  where  alluvial 
deposits  occur,  or  where  artificial  irrigation  is  possible.  There 
the  soil  is  often  remarkably  fertile,  but  the  un watered  areas 
are  only  suited  to  pasture. 

As  before  mentioned  there  are  certain  limited  areas  in  the 
British  Isles  where  wind  transport  plays  a  part  in  the  formation 
of  superficial  deposits.  Where  glacial  sands  are  abundant,  as 
in  parts  of  Norfolk  and  Suffolk,  they  are  often  much  blown 
by  wind  and  it  is  believed  that  the  loamy  soils  of  Holderness 


136  SUPERFICIAL  DEPOSITS  [CH.  v 

were  partly  formed  by  wind-transport  of  the  finer  argillaceous 
portions  of  boulder-clay. 

One  of  the  best  known  of  all  wind-borne  formations  in 
temperate  regions  is  the  Loess,  which  covers  such  large  areas 
in  central  Europe  and  in  Asia,  from  the  shores  of  the  English 
Channel  to  China.  It  is  probable  that  not  all  the  deposits 
included  under  the  name  of  Loess  were  formed  in  the  same 
way.  but  the  greater  part  is  generally  believed  to  be  due  to 
wind.  During  the  interglacial  periods  and  after  the  final 
departure  of  the  ice,  the  glaciated  area  and  the  regions  imme- 
diately bordering  it  must  have  been  largely  covered  by  glacial 
mud  of  very  fine  texture,  laid  down  by  streams  flowing  from  the 
ice.  This  mud  was  formed  from  the  fine  "rock  flour"  ground 
from  the  surface  of  the  rocks  by  the  passage  of  the  ice,  such  as 
may  be  seen  to-day  imparting  a  milky  tinge  to  all  rivers  flowing 
from  glaciers.  When  the  climate  became  warm  and  dry  the 
mud  was  converted  into  dust  and  transported  far  and  wide  by 
the  wind,  as  can  now  be  seen  in  central  Asia,  where  mountain 
chains  are  partly  or  almost  completely  buried  in  wind-borne 
dust.  The  Loess  is  normally  a  fine  yellowish  loam  of  very 
uniform  texture,  often  traversed  by  capillary  tubes  lined  with 
calcium  carbonate  and  due  to  roots.  These  partly  account 
for  the  facility  with  which  it  forms  vertical  cliffs  when  subjected 
to  stream-erosion.  The  Loess  often  forms  a  soil  of  remarkable 
fertility,  and  when  mixed  with  a  certain  proportion  of  peaty 
matter  it  gives  rise  to  the  well-known  Tchernozom  or  Black 
Earth  of  Russia  (see  also  p.  148). 

Loess  is  also  extensively  developed  in  certain  parts  of  the 
United  States,  especially  in  Minnesota  and  Dakota,  and  also 
in  Manitoba.  The  so-called  Adobe  of  the  Mississippi  valley  is 
very  similar  and  has  a  like  origin. 


CHAPTER   VI 

SOILS 

In  the  preceding  chapters  we  have  considered  in  a  general 
way  those  parts  of  geological  science  which  are  specially 
applicable  to  agriculture.  It  now  remains  to  combine  the 
information  there  contained  into  a  more  specialized  account  of 
the  geology  of  the  soil,  using  this  word  in  the  somewhat  narrow 
sense  as  generally  adopted  by  agricultural  writers  and  by 
practical  men.  In  this  sense  the  soil  is  taken  to  mean  in 
arable  land  the  cultivated  layer  down  to  the  depth  to  which 
ordinary  implements  penetrate,  and  in  pastures  down  to  the 
usual  limit  of  root  growth.  The  more  or  less  weathered  and 
disintegrated  portion  below  this  is  called  the  subsoil;  this 
part  is  not  disturbed  during  ordinary  agricultural  operations, 
but  crops  obtain  a  good  deal  of  their  food-supply  from  it. 
The  subsoil  passes  downwards  by  indefinite  gradations  into  the 
rock  below,  and  in  different  places  it  varies  very  greatly  in 
thickness,  being  sometimes  almost  completely  absent. 

It  has  already  been  made  apparent  that  soil-formation  is 
a  very  complicated  process,  involving  a  large  number  of 
different  agencies.  These  may  as  a  matter  of  convenience  be 
divided  into  three  groups,  chemical,  physical  and  biological, 
but  all  of  these  are  mutually  interdependent,  and  as  a  rule 
it  is  impossible  to  disentangle  the  effect  of  each. 

The  first  step  in  soil-formation  is  weathering ;  by  this  term 
we  understand  disintegration  of  rocks,  usually  accompanied 
by  chemical  and  mineralogical  changes  in  their  component 
minerals.  It  is  however  possible  for  a  rock  to  be  broken  up 
by  purely  physical  means,  without  any  chemical  change. 


138  SOILS  [CH. 

This  occurs  chiefly  in  arid  or  very  cold  regions,  where  water 
has  no  effect.  In  moist  climates  water  is  always  present  and 
brings  about  important  changes,  both  directly  by  solution  and 
indirectly  by  favouring  oxidation  and  chemical  reactions  in 
general.  The  other  principal  agents  of  weathering  have 
already  been  enumerated  in  Chapter  n.  The  general  effect  of 
all  these  taken  together  is  to  produce  on  the  surface  of  the  ground 
a  layer  of  loose  and  disintegrated  material,  which  in  itself 
constitutes  soil.  It  is  evident  that  in  such  a  case  the  nature 
and  composition  of  the  soil  is  controlled  to  a  large  extent  by 
the  original  composition  of  the  rocks  from  which  it  is  derived 
and  the  nature  of  the  chemical  processes  that  have  taken  place 
during  its  formation.  Chemical  processes  may  remove  some 
of  the  elements  originally  present,  but  in  general  they  cannot 
add  to  the  number,  except  when  aided  by  transport  of  material 
in  solution,  or  by  some  similar  method.  Soils  freshly  formed 
in  this  way  by  simple  weathering  of  rocks  are  therefore  often 
deficient  in  some  of  the  constituents  of  plant  food,  while  those 
that  are  present  may  not  exist  at  first  in  an  available  form. 
In  particular  the  supply  of  organic  matter  is  at  first  small  and 
will  gradually  increase  in  course  of  time  as  animals  and  plants 
die  and  decay  in  the  soil.  Hence  all  soils  both  natural  and 
cultivated  tend  to  get  richer  in  organic  matter  or  humus,  and 
consequently  in  nitrogen. 

In  the  majority  of  instances  the  weathered  material  does  not 
remain  in  exactly  its  original  position.  Nearly  always  more  or 
less  transport  takes  place,  as  before  described,  and  the  con- 
stituents of  many  soils  have  been  derived  entirely  from  distant 
sources.  Consequently  they  are  often  of  a  very  mixed  nature, 
and  have  no  necessary  relation  to  the  underlying  rock.  These 
facts  afford  a  basis  for  a  classification  into  sedentary  and  trans- 
ported soils.  As  examples  of  sedentary  soils  of  widely  differing 
type  we  may  refer  to  the  Chalk  soils  of  the  Downs  of  the  south 
of  England  and  the  peaty  soils  of  the  Fenland.  Good  examples 
of  the  transported  class  are  afforded  by  the  drift  deposits  of 
glacial  origin  and  the  alluvium  of  many  lowland  rivers.  In 
many  examples  of  the  latter  group  the  difference  between  soil 
and  subsoil  is  very  small. 


vi]  SOILS  139 

Modification  of  soils  by  wind-transport.  Some  30  years  ago 
attention  was  called  by  Mr  Clement  Reid1  to  some  anomalies 
in  the  character  of  certain  soils  when  compared  with  the 
substratum  on  which  they  lie.  It  has  long  been  recognized 
that  in  arid  countries  transport  of  dust  by  wind  can  give  rise 
to  important  superficial  deposits,  as  in  the  case  of  the  Loess, 
and  Darwin2  also  called  attention  to  this  possibility.  There  are 
certain  areas  in  the  east  of  England  where  the  rainfall  is  small, 
especially  in  spring,  and  at  this  season  strong  winds  are  prevalent, 
especially  from  the  east;  these  have  a  powerful  desiccating 
effect  on  the  superficial  deposits.  Mr  Reid  quotes  an  instance 
of  a  field  near  Cromer  which  was  sown  three  times  in  one  spring 
and  finally  left  fallow,  as  the  whole  of  the  soil  was  banked  up 
like  a  snowdrift  against  the  hedge.  Similar  effects  can  be  seen 
frequently  in  the  cultivated  parts  of  the  Breckland  of  western 
Norfolk,  where  it  is  worse  than  useless  to  cultivate  the  land 
on  a  windy  day.  In  course  of  time  this  process  of  sifting  by 
wind  must  bring  about  considerable  changes  in  the  character 
of  the  soil  by  removing  the  finer  particles,  the  clay  constituents, 
leaving  the  heavier  sand  behind  and  thus  making  the  soil  still 
lighter  (see  also  p.  133).  During  the  examination  of  the  super- 
ficial deposits  of  Holderness  and  Norfolk  by  the  Geological 
Survey  it  was  found  that  the  more  high-lying  areas  of  boulder- 
clay  carried  a  comparatively  light  loamy  soil,  while  large  areas 
of  almost  pure  sand  and  gravel  were  also  covered  by  fertile 
loams.  On  the  other  hand  in  the  hollows  the  soils  were  extra- 
ordinarily fine  in  texture,  even  where  the  substratum  was  not 
specially  heavy.  This  is  probably  due  to  removal  of  fine 
particles  of  clay  by  the  wind  from  exposed  places  with  deposition 
in  sheltered  spots.  Much  of  the  sandy  soil  covering  the  Chalk 
Wolds  of  Yorkshire  and  Lincolnshire  may  also  be  due  to  wind- 
transport. 

It  is  improbable  that  wind-transport  has  much  effect  on  the 
soils  in  the  western  and  northern  portions  of  our  islands,  or  in 
fact  in  any  region  where  the  rainfall  exceeds  30  inches  or 

1  Reid,  "Dust  and  Soils,"  Geol  Mag.  1884,  p.  165,  also  "Geology  of  Holder- 
ness,"  Mem.  Geol.  Surv.  1885,  p.  115. 

2  Darwin,  The  Formation  of  Vegetable  Mould,  p.  236. 


140  SOILS  [CH. 

thereabouts,  but  in  the  dry  regions  of  eastern  England  there 
can  be  no  doubt  that  it  has  an  important  influence  on  the  texture 
and  fertility  of  the  soil,  this  effect  on  the  whole  being  one  of 
amelioration,  making  light  soils  heavier  and  heavy  soils  lighter, 
thus  giving  rise  to  a  general  levelling-up.  It  is  only  injurious 
in  extreme  cases,  especially  where  the  soil  is  removed  bodily. 

Formation  of  "  pans  "  in  soils.  Almost  all  writers  on 
agriculture  have  described  the  formation  in  soils  of  hard  layers, 
or  "pans,"  a  short  distance  below  the  surface,  but  few  of  them 
appear  to  have  realized  that  two  different  phenomena  are  here 
confused.  The  simplest  kind  of  pan  is  of  purely  mechanical 
origin,  and  is  confined  to  soils  that  have  been  long  under 
cultivation.  The  manner  of  formation  is  as  follows :  year  after 
year  the  plough  penetrates  to  the  same  depth,  turning  over  the 
upper  layer,  from  7  to  9  inches  thick,  according  to  the  character 
of  the  soil  and  the  ease  of  cultivation,  but  tending  to  consolidate 
the  layer  below,  which  has  to  bear  the  weight  of  the  plough, 
and  must  become  somewhat  more  solid  every  time  it  is  com- 
pressed by  the  weight  of  a  horse  or  of  an  implement.  Ultimately 
this  is  pressed  and  trampled  hard,  and  it  is  to  be  noted  that  in 
this  country  the  loosening  effect  of  winter  frost  does  not  pene- 
trate more  than  a  few  inches  into  the  soil.  This  hardened  and 
compressed  layer  forms  a  barrier  to  the  free  passage  of  air, 
water  and  the  roots  of  plants.  A  pan  of  this  sort  is  formed  most 
readily  in  clay  soils,  and  is  unknown  in  light  sands.  The  obvious 
remedy  is  occasionally  to  cultivate  the  soil  to  an  extra  depth, 
or  to  employ  the  subsoil  plough;  these  are  unfortunately 
somewhat  costly  remedies,  but  will  often  repay  the  extra 
expense  and  labour  involved. 

The  other  kind  of  pan,  the  Ortstein  of  the  Germans,  is  of 
somewhat  more  complicated  origin,  and  is  due  primarily  to 
the  circulation  of  soluble  material  in  the  soil.  Such  pans  are 
most  commonly  formed  in  soils  rich  in  humus,  in  regions  of 
abundant  seasonal  rainfall,  especially  where  the  soil  is  of  a 
peaty  nature.  In  the  upper  layers  of  such  soils  the  vegetable 
matter  undergoes  a  special  kind  of  decomposition,  giving  rise 
to  humous  compounds  which  generally  have  an  acid  reaction, 
the  so-called  humic  acids.  By  the  weathering  of  minerals  also 


vi]  SOILS  141 

many  soluble  substances  are  formed,  chiefly  salts  of  iron,  lime, 
magnesia  and  other  bases.  These  are  carried  down  in  solution 
into  the  subsoil  and  underlying  rocks,  so  long  as  the  general 
movement  of  water  is  downwards.  Where  there  is  free  drainage 
at  deep  levels,  that  is,  where  the  underlying  rocks  are  porous, 
all  these  soluble  matters  are  completely  removed  and  there  is 
no  pan-formation.  The  chief  result  is  a  bleaching  of  the  lower 
layers  of  the  soil  and  of  the  subsoil.  Such  bleached  layers  are 
often  to  be  seen  below  thin  peaty  soils  resting  on  a  sandy 
foundation.  But  in  many  cases  the  water  is  not  carried  away 
by  free  drainage,  being  held  up  by  an  impermeable  layer,  and 
the  soluble  materials  tend  to  accumulate  in  the  soil  water. 
During  the  drier  season  of  the  year  the  downward  movement 
of  rain-water  ceases ;  on  the  other  hand  the  ground- water  from 
the  lower  layers  begins  to  move  upwards  to  replace  that  lost 
by  evaporation  from  the  surface;  hence  the  soluble  materials 
also  have  an  upward  tendency.  When  the  top  layers  of  the 
soil  are  dry,  atmospheric  oxygen  is  abundant  in  the  soil  and 
the  solutions  as  they  rise  are  oxidized ;  this  oxidation  leads  to 
a  precipitation  of  various  compounds,  especially  iron  oxides 
and  hydroxides  and  humus,  in  the  soil,  forming  a  solid  layer, 
usually  at  a  depth  of  15  to  20  inches,  this  being  the  depth 
commonly  reached  by  atmospheric  oxygen. 

The  essence  of  the  whole  process  lies  in  the  fact  that  when 
soils  rich  in  humus  are  saturated  with  water,  reduction  is  taking 
place,  whereas  when  the  soils  are  free  from  interstitial  water, 
and  therefore  full  of  air,  oxidation  is  dominant,  destroying  the 
humic  acids  and  liberating  the  iron,  lime  and  magnesia.  Under 
such  conditions  the  liberated  substances,  oxides  and  hydrated 
oxides  of  iron,  calcium  carbonate,  etc.,  may  form  either  nodules 
and  concretions  in  the  soil,  or  in  some  cases  a  continuous  layer 
or  pan.  The  composition  of  these  solid  segregations  varies 
widely ;  in  some  soils  they  consist  of  pure  or  nearly  pure  calcium 
carbonate,  such  as  the  kankar  of  the  Indian  cotton  soils,  and 
the  surface  limestones  of  South  Africa.  More  commonly 
however  they  are  composed  of  varying  mixtures  of  iron  hydrates 
with  organic  material  (humus),  the  relative  proportions  varying 
with  the  humidity  of  the  climate.  In  damp  regions  the  pan 


142  SOILS  [CH. 

may  contain  as  much  as  17  per  cent,  of  organic  matter;  in  arid 
regions  the  amount  generally  varies  between  1  and  3  per  cent.1 

The  Ortstein  of  German  authors,  which  is  so  common  and 
characteristic  in  the  sandy  soils  of  north  Germany  and  Kussia, 
may  be  described  as  a  bed  of  sandstone  cemented  by  humus 
rendered  insoluble  and  hard  by  oxidation.  The  published 
analyses  of  this  substance  do  not  show  any  excessive  amount 
of  iron,  but  alumina  is  fairly  abundant.  When  left  exposed 
to  the  air  the  Ortstein  quickly  decomposes  to  a  loose  brown 
sand,  owing  to  decomposition  of  the  cementing  material,  and 
this  breaking  up  is  much  facilitated  by  frost. 

The  "pans"  formed  in  the  soils  of  this  country  appear  to 
consist  in  the  main  of  hydrated  oxide  of  iron  (limonite).  This 
is  found  both  in  sands  and  clays  which  are  imperfectly  drained 
and  therefore  waterlogged.  The  depth  at  which  it  occurs  is 
doubtless  controlled  partly  by  the  lower  limit  of  cultivation,  as 
previously  explained,  and  partly  by  the  distance  to  which  air 
can  penetrate  freely  to  cause  oxidation.  Pan-formation  in 
this  country  is  a  subject  requiring  much  more  complete  and 
careful  investigation  than  it  has  yet  received,  and  in  particular 
careful  analysis  of  a  considerable  number  of  samples  from 
different  localities  is  most  desirable.  It  has  been  suggested 
with  a  considerable  degree  of  probability  that  bacteria,  especially 
the  iron-bacteria  of  Vinogradsky,  may  play  a  part  in  the  process, 
but  on  this  point  no  certain  information  is  available2. 

In  certain  arid  regions  where  the  soil  is  rich  in  sodium 
carbonate  (black  alkali  soils)  a  hard  pan  is  often  found,  some- 
times so  hard  as  to  be  with  difficulty  broken  with  a  hammer. 
The  existence  of  this  pan  is  due  to  the  fact  that  sodium  carbonate 
prevents  flocculation  of  finely  divided  clay  substance,  but  on 
the  other  hand  converts  it  into  a  hard  horny  mass,  quite 
impervious  to  water.  This  effect  can  be  counteracted  by 
neutralization  of  the  alkali  by  treatment  of  the  soil  with 
gypsum3. 

1  Treitz,  'Was  1st  Verwitterung ? "  Comptes  Eendus  l^e  Conference  Internal. 
Agrogeol.  Budapest,  1909,  p.  138.     Ramann,  Bodenlcundz,  Berlin,  1911,  p.  204. 

2  Hall,  The  Soil,  2nd  edition,  London,  1910,  p.  285. 

3  Hilgard,  Soils,  New  York,  1906,  p.  62 


VI] 


SOILS 


143 


Since  the  information  on  the  subject  of  pans  contained  in 
English  agricultural  literature  is  very  scanty,  here  are  appended 
two  analyses  of  such  material  carried  out  by  the  author.  It 
is  seen  that  by  far  the  most  abundant  constituents,  besides 
sand,  are  organic  matter  and  iron  oxide,  although  the  latter  is 
in  considerably  less  proportion  than  is  generally  believed  to  be 
the  case.  In  the  first  instance,  a  pan  from  Norfolk  with  a 
highly  ferruginous  appearance,  the  iron  oxide  only  amounts  to 
a  little  over  4  per  cent. ;  nevertheless  this  pan  is  very  hard  and 
presents  a  serious  barrier  to  successful  cultivation.  The  pan 
from  Cheshire,  locally  called  "fox-bench,"  is  very  dark  in  colour, 
almost  black,  and  this  feature  is  correlated  with  the  very  high 
proportion  of  humus.  The  analyses  were  carried  out  on  air- 
dried  material ;  other  constituents  though  possibly  present 
were  in  amounts  too  small  to  be  determinable. 


Organic  matter  

Sand  and  silicates  (insoluble) 

Iron  oxide 

Lime     ... 

Magnesia 

Phosphoric  acid          


The  characters  of  soils.  The  agricultural  value  of  a  soil 
depends  on  a  large  number  of  factors,  only  some  of  which  are 
within  the  province  of  the  geologist.  Pre-eminent  among  these 
are  its  mineralogical  composition  and  the  state  of  division  of 
its  particles;  on  the  former  subject  little  work  has  been  done, 
and  a  large  field  for  research  is  here  open,  especially  with  regard 
to  the  mineral  character  of  the  smaller  particles.  On  the  other 
hand  mechanical  analysis  by  various  methods  has  been  made 
the  basis  of  soil-investigations  by  workers  in  many  places,  in 
this  country  for  example  at  Rothamsted  and  at  Cambridge. 
Owing  to  want  of  space  the  methods  employed  cannot  be 
described  here.  The  study  of  the  biology  of  the  soil  is  a  rather 
large  and  controversial  subject,  on  which  also  much  remains 


Brown  ferru- 
ginous pan. 
Little  Snoring, 
Norfolk 

Black  pan 
(Fox-bench) 
Delamere  Forest, 
Cheshire 

6-50 

13-04 

87-13 

83-55 

4-31 

2-82 

•90 

trace 

— 

•20 

trace 

— 

98-84 

99-61 

144  SOILS  [CH. 

to  be  done.,  especially  in  the  direction  of  bacterial  investigation. 
The  epoch-making  discovery  by  Hellriegel  and  Willfarth  in 
1887  of  the  symbiotic  nitrogen-fixing  bacteria  in  the  root 
nodules  of  the  Leguminosae  opened  up  a  wide  field  of  enquiry, 
of  immense  practical  value,  and  furnished  a  scientific  explana- 
tion of  the  long-observed  fact  that  the  leguminous  crops  enrich 
the  land  rather  than  impoverish  it. 

In  this  book  no  attempt  is  made  to  deal  with  certain 
properties  of  the  soil  of  an  essentially  physical  nature,  such 
as  texture,  porosity,  absorptive  power  for  water  and  so  on. 
These  subjects,  though  of  much  practical  importance,  are  not 
strictly  geological,  and  are  treated  fully  in  many  agricultural 
works1. 

Classification  of  soils.  The  ordinary  practical  classification 
is  a  somewhat  rough  and  ready  one,  founded  on  the  agricultural 
character  of  the  soil,  its  more  obvious  characteristics  and  the 
comparative  ease  or  difficulty  of  working,  rather  than  on  any 
very  precise  examination  of  its  constituents.  The  practical 
farmer  designates  his  soils  as  heavy  or  light,  according  to  the 
difficulty  or  ease  of  working,  and  irrespective  of  actual  specific 
gravity ;  in  point  of  fact  a  cubic  foot  of  sand  is  heavier  than  a 
cubic  foot  of  clay.  Here  tenacity  is  really  the  determining 
factor,  not  weight. 

A  useful  working  classification  of  this  kind  is  the  following, 
which  divides  soils  into  six  classes,  namely,  gravelly,  sandy, 
loamy,  marly,  clay  and  peaty  soils.  Most  of  these  terms 
explain  themselves. 

A  gravelly  soil  is  one  containing  abundance  of  stones, 
imparting  a  distinct  character  to  it,  while  the  term  sandy 
certainly  needs  no  explanation  here.  A  loam  is  a  soil  containing 
sand  and  clay  in  approximately  equal  proportions,  and  for 
most  purposes  this  is  the  best  sort  of  soil,  being  rich  in  plant 
food  and  also  free  working.  A  marl  is  a  soil  containing  a  fair 
proportion  of  calcium  carbonate  in  addition  to  clay;  marls  are 
often  rather  heavy,  pasty  soils.  Peaty  soils  are  those  in  which 
organic  matter  is  present  to  an  excessive  degree;  they  are 

1  See  especially  Hall,  The  Soil,  2nd  edition,  London,  1910,  Chapters  m-v. 
This  book  also  contains  an  excellent  chapter  on  mechanical  analysis. 


vi]  SOILS  145 

generally  dark  in  colour  and  often  easy  to  work,  though  soft 
in  wet  weather. 

Besides  this  rough  and  ready  every-day  classification, 
numerous  attempts  have  been  made  to  draw  up  schemes  based 
on  the  geological  origin  of  soils,  on  the  climatic  conditions 
controlling  their  formation,  and  on  many  other  factors.  Some 
of  the  most  elaborate  of  these  schemes  are  due  to  Russian 
workers,  such  as  Sibirtzev  and  Glinka.  Many  others  also  exist 
in  German  and  American  agricultural  literature. 

In  the  most  satisfactory  of  these  classifications  the  basis  is 
climatic,  being  really  founded,  though  this  is  not  always 
explicitly  stated,  on  the  primary  subdivision  of  the  globe  into 
climatic  zones.  Here  as  in  other  cases  it  is  necessary  to  bear 
in  mind  that  an  increase  of  altitude  may  produce  the  same 
effect  as  an  increase  of  latitude,  i.e.  highlands  in  the  tropics 
may  have  a  temperate  climate  and  the  highest  mountain  ranges 
all  over  the  world  are  more  or  less  arctic.  One  of  the  most 
important  of  all  the  controlling  factors  is  the  amount  of  the 
rainfall,  and  the  soils  of  arid  regions  show  very  special 
characters,  which  are  not  to  a  great  extent  dependent  on 
latitude  or  altitude,  not  so  much  so  at  any  rate  as  in  moist 
climates. 

Although  climate  is  affected  thus  by  many  local  variations 
of  elevation,  aspect  and  so  forth,  if  a  sufficiently  large  area  is 
taken  it  is  possible  to  draw  up  generalizations,  and  to  divide 
a  country  into  regions  characterized  by  soils  of  a  more  or  less 
definite  character.  The  British  Isles  are  too  small  in  area,  and 
built  up  of  too  many  varieties  of  rocks,  to  afford  a  satisfactory 
basis,  and  the  matter  is  rendered  still  more  difficult  by  the 
varying  relief  of  the  land  and  the  great  local  differences  in 
rainfall.  To  obtain  satisfactory  results  we  must  turn  to  some 
large  area  of  uniform  relief  and  geological  structure,  preferably 
some  way  from  the  sea  and  showing  a  steady  progressive 
change  of  climate  from  south  to  north.  Perhaps  the  best  and 
most  satisfactory  example  is  to  be  found  in  European  Russia. 
This  great  country  stretches  through  some  30°  of  latitude,  or 
over  2000  miles  from  north  to  south,  and  nearly  as  much  from 
east  to  west;  it  is  in  the  main  one  vast  level  plain  from  the 
R.A.G.  10 


146  SOILS  [CH. 

shores  of  the  Baltic  to  the  Urals,  and  the  geological  structure 
is  remarkably  simple  and  uniform.  Owing  to  causes  which 
need  not  here  be  discussed  the  climatic  belts  do  not  run  exactly 
east  and  west,  but  trend  from  south-west  to  north-east,  and  on 
the  whole  the  soil-belts  follow  these  very  closely.  The  extreme 
north  is  exceedingly  cold,  in  fact  almost  arctic  in  character, 
while  in  the  south-east  the  rainfall  is  small  and  the  summer 
climate  hot.  This  part  belongs  rather  to  the  arid  zone,  while 
the  middle  portion  is  more  temperate,  though  everywhere  the 
winters  are  cold. 

From  a  detailed  study  of  the  soil-types  of  Russia  Sibirtzev1 
has  drawn  up  a  zonal  classification,  and  he  finds  that  the 
arrangement  of  the  soil-belts  shows  a  rough  parallelism  to  the 
climatic  zones.  In  all  six  principal  types  can  be  recognized; 
besides  these  there  are  occasional  patches  of  so-called  interzonal 
soils,  in  places  where  local  peculiarities  of  climate  or  of  topo- 
graphy have  given  rise  to  well-marked  special  conditions. 
Again  along  the  chain  of  the  Urals  and  elsewhere  are  found 
certain  superficial  deposits,  not  true  soils  in  the  ordinary 
sense,  but  geological  formations  that  may  be  considered 
independent  of  climate.  The  nature  of  these  will  be  considered 
later. 

The  classification  is  based  on  genetic  principles;  that  is  to 
say  that  under  identical  climatic  conditions  rock-disintegration 
can  efface  original  differences  between  rocks  and  gives  rise  to 
similar  products.  Hence  a  particular  soil-type  is  the  result 
of  its  geographical  environment,  that  is,  of  the  conditions 
dominant  in  its  zone.  In  each  zone  there  are  local  varieties 
but  all  possess  certain  characters  in  common. 

Sibirtzev's  classification,  as  applied  to  Russia,  may  be 
arranged  as  follows  in  a  tabular  form2. 

1  Sibirtzev,  "  fitude  des  Sols  de  la  Russie,"  Congres  Geologique  Internationale 
St  Petersburg,  1897,  Report,  p.  73.     See  also  Glinka,  Die  Typen  der  Bodenbildung, 
Berlin,  1914  (a  German  translation  of  the  author's  Russian  lectures,  with  a 
good  coloured  soil  map  of  Russia). 

2  In  the  original  publication  the  first  type  of  zonal  soils  is  laterite      This 
however    does    not    occur    in    Russia,   and    has   here    been    omitted,    to   be 
treated   along  with   other  important  types  also   absent  from  the  list  given 
by  Sibirtzev. 


vi]  SOILS  147 

A.  Zonal  soils. 

1.  Aeolian  soils. 

2.  Dry  steppe  soils. 

3.  Tchernozom  or  black  earth. 

4.  Grey  forest  soils. 

5.  Podzols. 

6.  Tundra  soils. 

B.  Interzonal  soils. 

1.  Salt  soils. 

2.  Redzina  or  Borovina  soils. 

3.  Marsh  soils. 

C.  Azonal  soils  (or  surface  deposits). 

1.  Screes,  moraines,  river  gravels,  sand  dunes. 

2.  Alluvium. 

(1)  Aeolian  or  dust  soils.     This  group  includes  the  soils 
that  are  formed  from  the  Loess  and  other  wind-transported 
deposits  that  are  so  abundant  in  the  dry  regions  of  the  arid 
belt.     (For  an  account  of  the  origin  of  the  Loess  see  p.  136.) 
There   is   usually   little   difference   between   soil   and   subsoil, 
chiefly  owing  to  the  small  proportion  of  humus,  which  is  usually 
less  than  1  per  cent.,  rarely  rising  to  2  or  3  per  cent.     The  soils 
are   commonly   very   fine   in    texture,    somewhat   loamy   and 
generally  of  a  yellow  or  even  orange  colour.     When  irrigated 
they  are  often  fairly  fertile,  but  commonly  suffer  from  want  of 
water,  owing  to  the  climate. 

Soils  of  this  class  have  a  wide  distribution  in  the  Trans- 
Caspian  region  in  Turkestan,  and  in  many  parts  of  central  Asia 
and  China.  They  are  also  found  in  the  Great  Basin  region  of 
the  western  and  south-western  United  States.  In  the  southern 
hemisphere  the  soils  of  the  Karroo  region  of  Cape  Colony  may 
also  be  referred  here. 

(2)  Dry  steppe  soils   (the  chestnut- brown  soils  of  Doku- 
tchaiev)    occupy    a    considerable   space   in    the   south-eastern 
corner  of  European  Russia,  between  the  Ural  and  the  Volga, 
along  the  shores  of  the  Black  Sea,  and  in  parts  of  the  Crimea. 
In  this  region  the  annual  rainfall  ranges  from  7  to  10  inches 

10—2 


148  SOILS  [CH. 

and  vegetation  is  scanty.  The  subsoil  is  mainly  soft  Tertiary 
or  post-Tertiary  clays,  often  with  rock-salt  and  gypsum. 
Patches  of  interzonal  saline  soils  are  common  in  depressions. 
Towards  the  north  there  is  a  gradual  transition  to  the  black- 
earth  soils  of  the  next  zone. 

With  these  may  be  compared  the  soils  of  the  "Desiertos" 
of  eastern  Spain,  and  some  soils  in  California,  Colorado  and 
New  Mexico,  as  well  as  parts  of  the  Orange  Free  State,  and  the 
Transvaal  in  South  Africa. 

(3)  The  Black  Earth  or  Tchernozom.  This  soil-type  is 
of  enormous  agricultural  importance,  constituting  the  great 
wheat-growing  area  of  Russia  and  other  neighbouring  countries 
of  south-eastern  Europe.  The  black  earth  region  mainly  forms 
one  vast  plain,  with  occasional  ravines  and  hollows  and  in 
these  alone  are  trees  found  to  any  great  extent.  The  rainfall 
is  small,  from  10  to  13  inches  only,  and  serious  droughts  are 
frequent.  Hence  the  climatic  conditions  are  well  marked  and 
have  had  an  important  influence  on  the  formation  of  the  soil. 

The  origin  of  the  Tchernozom  is  a  subject  that  has  given 
rise  to  much  controversy  and  the  literature  of  the  subject  is 
very  large1.  From  an  exhaustive  study  of  the  characters  and 
distribution  of  the  soil  Dokutchaiev  concludes  that  it  always 
shows  a  close  genetic  relationship  with  the  underlying  rocks, 
and  is  not,  as  was  formerly  believed,  a  transported  deposit  of 
marine  or  glacial  origin.  Tchernozom  can  be  formed  from 
various  rocks,  especially  Loess,  Chalk,  Jurassic  clays,  or 
weathered  granite.  This  affords  a  very  clear  illustration  of  the 
general  principle  that  under  determinate  conditions  similar  soils 
may  arise  from  very  different  rocks  (i.e.  the  zonal  principle  of 
Sibirtzev).  It  is  not  formed  in  tree-covered  regions,  but  only 
under  grassy  steppes.  The  soil  has  a  very  constant  thickness, 
rarely  if  ever  exceeding  5  feet,  and  the  structure  is  markedly 
granular. 

As  its  name  implies  Tchernozom  is  characterized  by  a  very 
dark  colour,  usually  black,  but  occasionally  shading  into  grey 
or  brown  of  a  dark  shade.  Spots  and  concretions  of  calcium 

1  For  a  summary  of  the  Russian  publications  on  the  subject,  see  Kossowitsch, 
Die  Schwarzerde,  Vienna,  1912,  a  German  translation  of  a  Russian  monograph. 


VI] 


SOILS 


149 


and  magnesium  carbonates  are  common,  often  occurring  in 
worm- burrows.  The  upper  layers  of  the  soil  are  exceedingly 
rich  in  humus,  the  amount  sometimes  rising  to  16  per  cent, 
and  averaging  about  10  per  cent.  Sometimes  layers  rich  in 
humus  are  also  observed  deep  down  in  the  subsoil.  These  are 
no  doubt  analogous* to  the  Ortstein  of  the  northern  soils  (see 
p.  140).  The  humus  appears  to  be  derived  wholly  from  the 
growth  and  decay  of  vegetation,  especially  the  roots  of  grasses. 
The  following  table  from  the  work  of  Kossovitch,  already 
cited,  shows  the  chemical  composition  of  two  representative 
samples  of  Tchernozom,  one  rich  in  humus,  the  other  rather 
poorer. 


Humus 

Water     

Phosphorus  pentoxide 
Silica       ...         ...      '  .; 

Alumina 
Ferric  oxide 
Lime 
Magnesia 

Potash 

Soda 


Tchernozom, 
Bobrov,  Gouv. 
Voronesh 

Tchernozom, 
Nikolaievsk,  Gouv. 
Samara 

11-73 

7-84 

2-94 

2-87 

•32 

\2S 

59-93 

64-40 

12-61 

13-12 

5-20 

4-62 

2-32 

1-41 

1-74 

1-58 

2-11 

2-22 

1-05 

1-33 

99-95 


99-89 


The  fertility  of  this  soil  is  extraordinary;  it  appears  to  be 
able  to  grow  heavy  crops  of  wheat  and  maize  for  an  indefinite 
time  under  an  extensive  system  of  farming,  with  little  or  no 
manure. 

The  general  distribution  of  the  Tchernozom  in  European 
Russia  is  simple ;  it  forms  a  broad  band  stretching  W.S.W.  and 
E.N.E.,  from  the  western  frontier  of  Russia,  across  the  basins 
of  the  Dniepr,  Don  and  Volga,  towards  the  southern  end  of  the 
Urals,  where  an  interruption  occurs.  It  covers  also  a  wide 
extent  in  Siberia,  in  the  Governments  of  Tobolsk  and  Tomsk, 
even  occurring  in  patches  as  far  as  the  basin  of  the  Amur. 
The  total  area  occupied  by  it  in  European  Russia  is  estimated 


150  SOILS  [CH. 

at    about    350,000    square   miles,    and   it    also    extends    into 
Rumania1,  Galicia  and  Hungary. 

With  the  Tchernozom  may  be  compared  the  "black  cotton 
soil"  of  India  (p.  160),  some  of  the  prairie  soils  of  North  America 
(p.  152),  and  the  black  soil  of  parts  of  Argentina. 

(4)  Grey  forest  soils.     The  soils  of  this*type  form  a  narrow 
but  fairly  continuous  band  to  the  north  of  the  Tchernozom, 
running  from  Lublin  and  Volhynia  to  Kama  and  Viatka.     The 
district  was  probably  once  open  steppe,  but  is  now  covered  by 
forests.     The  soil  lies  on  moraines  and  on  argillaceous  sediments. 
Though  once  doubtless  rich  in  humus,  this  has  been  leached  out 
by  vegetation  and  now  varies  from  3  to  6  per  cent.     The  soils 
are  grey  in  colour  and  granular  in  structure.     The  thickness  is 
small,  rarely  exceeding  10  inches;    the  subsoil  is  very  similar 
and  seldom  more  than  a  foot  thick. 

Grey  forest  soils  are  known  to  occur  in  Siberia,  and  they 
may  also  exist  in  Galicia,  Hungary  and  in  some  German  forests, 
as  well  as  along  the  southern  border  of  the  Canadian  forests, 
but  little  is  known  about  them  outside  Russia. 

(5)  Podzol.     This  general   designation   is  now   commonly 
applied  to  a  soil-type  that  covers  enormous  areas  in  European 
Russia,  at  least  two-fifths  of  the  entire  country,  as  well  as  most 
of  Siberia  except  the  extreme  north.     The  climate  of  this  belt 
is  fairly  moist,  with  very  cold  winters  and  the  country  in  its 
natural  state  is  covered  with  coarse  grass,  brushwood,  scattered 
trees  and  occasionally  marshes.     Beds  of  peat  are  fairly  common. 
The  podzols  are  in  the  main  similar  to  the  German  bleached 
soils  (Bleicherde),  the  principal  characteristic  being  the  leaching 
out  of  the  soluble  constituents  from  the  upper  layers,  including 
the  iron  and  phosphoric  acid.     These  dissolved  constituents 
are  commonly  reprecipitated  some  distance  below  the  surface, 
forming  a  pan  (Ortstein).     The  soils  are  white  or  grey  in  colour 
and  very  siliceous;  they  are  very  infertile  unless  well-manured, 
owing  to  the  removal  of  plant  food  in  solution.     The  subsoil 
is  commonly  of  morainic  origin. 

Podzols  are  in  the  main  sandy  soils,  very  like  the  bleached 

1  Murgoci,    "Die   Bodenzonen    Rumaniens,"    C.    E.    l^'e   Conf.    Internal. 
Agrogdol.  Budapest,  1909. 


vi]  SOILS  151 

sands  so  often  found  underlying  the  heather  in  central  Europe 
and  in  the  east  of  England,  wherever  owing  to  small  rainfall  or 
other  causes  the  conditions  are  unfavourable  for  the  growth  of 
peat.  The  name  is  more  strictly  applicable  to  the  bleached 
subsoil,  as  distinguished  from  the  upper  layer  where  humus  is 
more  abundant.  Where  the  latter  layer  is  absent  the  land  is 
extraordinarily  barren. 

(6)  Tundra.  From  the  agricultural  standpoint  this  can 
scarcely  be  regarded  as  a  true  soil.  The  tundra  is  really  a 
gigantic  peat  bog  which  is  permanently  frozen  at  a  depth  greater 
than  3  or  4  feet;  its  characters  and  origin  are  described  in 
Chapter  v  (p.  120).  It  is  confined  to  the  extreme  north  of 
Russia,  Siberia  and  North  America  and  is  entirety  uncultivated. 

Among  the  interzonal  soils  of  Russia  two  or  three  types 
only  need  brief  mention.  These  are  the  saline  soils  of  the 
south-east,  the  Redzina  soils  of  Poland,  and  the  marsh  soils. 

(1)  The  saline  soils  or  Solontzy  are  found  in  the  south- 
eastern corner  of  European  Russia  in  the  Trans-Caspian  territory 
and  in  Turkestan.     They  also  form  occasional  islands  in  the 
Tchernozom  belt.     The  soil  is  generally  black  above  and  grey 
below,  and  at  times,  when  evaporation  has  been  active,  the 
surface  is  powdered  with  salt  crystals.     The  subsoil  is  usually 
a  stiff  brown  or  red  saline  clay.     The  salts  present  in  the  soil 
are  those  usually  associated  with  salt  lakes,  especially  chlorides 
and  sulphates  of  sodium,  magnesium  and  calcium.     Since  the 
more  important  varieties  of  saline  and  alkaline  soils  are  treated 
elsewhere  (see  p.  156),  it  is  unnecessary  to  pursue  the  subject 
here. 

(2)  The  Redzina  or  Borovina  soils  of  Poland.     These  soils 
are  of  rather  unusual  character,  since  they  are  of  a  humous  or 
peaty  nature,  though  lying  on  Chalk  or  limestone.    The  upper- 
most layer  is  dark  grey  in  colour,  owing  to  the  humus;    then 
comes  usually  a  layer  of  clay  or  marl  with  the  unaltered  cal- 
careous rock  below.     The  origin  and  affinities  of  these  soils 
have  not  yet  been  sufficiently  explained,  but  their  analogues 
might  be  sought  in  some  of  the  East  Anglian  heaths  lying  on 
Chalk. 

(3)  Swamp  soils.     These  soils  are  almost  exactly  similar 


152  SOILS  [CH. 

to  the  fen  peat  of  England  and  Germany;  according  to  the 
published  accounts,  the  plants  that  take  part  in  their  formation 
belong  mainly  to  the  same  species  of  rushes,  sedges,  grasses, 
Butomus,  Sagittaria,  etc.,  as  are  found  in  the  English  Fenland. 
A  detailed  description  is  obviously  unnecessary  (see  also  p.  119). 

The  azonal  soils  of  Sibirtzev's  classification  are  in  the  main 
surface  deposits,  which  can  scarcely  be  reckoned  as  soils  in  the 
ordinary  sense.  They  are  found  in  regions  where  special  local 
conditions  have  given  rise  to  extensive  superficial  deposits  of 
various  kinds,  largely  independent  of  climate  and  of  geological 
structure.  Here  for  example  may  be  included  the  screes  of 
mountain  ranges  and  of  stony  deserts,  boulders  of  disintegration, 
especially  those  lying  on  outcrops  of  igneous  rock,  bare  rock 
surfaces  of  any  kind,  including  some  limestone  plateaux  with 
underground  drainage,  the  gravel  beds  of  rivers,  the  sand-hills 
of  the  sea-shore,  the  moraines,  eskers,  drumlins,  gravels  and 
sands  of  glaciated  regions  and  others.  In  this  class  also  may 
be  placed  the  alluvial  deposits  of  rivers  and  deltas,  and  the 
lagoon  deposits  of  the  sea-shore.  All  of  these  when  weathered 
give  rise  to  more  or  less  soil,  those  of  the  alluvial  class  often 
being  extremely  fertile.  The  origin  of  all  these  deposits  is 
treated  in  Chapter  v.  It  is  evident  that  the  soils,  if  any,  derived 
from  them  will  show  great  variety,  owing  to  local  conditions, 
varying  with  the  lithological  character  of  the  deposits  themselves, 
so  that  no  general  account  can  be  given. 

Soil-zones  in  North  America.  As  already  remarked  the 
majority  of,  if  not  all,  the  Russian  soil-types  here  enumerated 
can  be  identified  in  North  America.  Here  owing  to  the  dis- 
position of  the  climatic  zones,  the  soil-belts  trend  from 
south-east  to  north-west,  the  hottest  and  driest  region  being  in 
Arizona,  Nevada,  Utah  and  southern  California,  where  the 
average  temperature  for  the  month  of  July  exceeds  90°  F. 
Here  are  found  representatives  of  the  saline  and  dry  steppe 
soils.  Aeolian  soils,  Loess  or  Adobe  are  common  in  Colorado, 
Utah  and  Wyoming,  while  the  black  prairie  soil,  the  represent- 
ative of  the  Tchernozom,  covers  an  enormous  area  in  Montana, 
Dakota,  Nebraska,  Iowa,  Wisconsin,  Minnesota,  Kansas, 
Missouri,  Arkansas,  and  Oklahoma.  It  also  extends  widely 


vi]  SOILS  153 

in  the  corn-growing  regions  of  Canada,  in  Manitoba,  Saskatche- 
wan and  Alberta.  Grey  forest  soils  exist  in  the  northern  part 
of  the  American  prairies,  bordering  on  the  forest  zone.  In 
the  latter,  soils  of  the  podzol  type  have  a  wide  extension,  while 
in  the  far  north  the  tundra  region  is  very  extensive  in  Alaska 
and  east  of  the  Mackenzie  river.  Thus  it  appears  that  in  western 
America  the  zonal  distribution  of  soils  is  quite  as  strongly 
marked  as  in  Russia.  In  eastern  America  however  the  soils 
appear  to  show  more  resemblance  to  those  of  western  Europe. 

Although  the  soils  of  Russia  and  western  America  afford 
perhaps  the  best  examples  of  zonal  arrangement,  nevertheless 
several  of  the  most  important  and  widespread  types  are  there 
unrepresented ;  some  notable  examples  are  the  lateritic  and  red 
soils  of  the  tropical  and  subtropical  regions  and  above  all  the 
ordinary  arable  and  meadow  soils  of  western  Europe  and  the 
eastern  states  of  America.  These  and  others  it  is  now  necessary 
to  treat  in  some  detail. 

Lateritic  soils.  The  origin  of  laterite  has  already  been 
discussed  (pp.  57  and  116).  It  was  there  pointed  out  that  laterite 
results  from  a  special  kind  of  chemical  weathering,  the  dominant 
feature  being  a  hydrolytic  decomposition  of  silicates,  with 
great  loss  of  free  silica  and  of  alkalies,  lime  and  magnesia,  and 
a  consequent  increase  by  concentration  of  iron  and  alumina. 
This  process  is  if  anything  still  more  accentuated  in  the  super- 
ficial layers,  and  owing  to  the  generally  abundant  rainfall  of 
the  tropics  there  is  much  solution  and  oxidation  and  little 
accumulation  of  humus.  Laterite  soils  are  therefore  generally 
red  in  colour,  and  are  often  strikingly  deficient  in  some  of  the 
elements  of  plant  food.  Laterite  formation  is  clearly  deter- 
mined by  climate,  and  appears  to  be  best  developed  where 
there  is  a  marked  alternation  of  wet  arid  dry  seasons.  According 
to  all  authorities  laterite  formation  is  the  distinctive  and 
characteristic  geological  process  of  the  tropical  zone. 

The  principal  constituents  of  laterite  soils  are  hydrated 
oxides  or  hydrates  of  iron  and  alumina,  together  with  a  con- 
siderable amount  of  unweathered  quartz.  In  India  especially 
calcareous  concretions  are  often  found  in  abundance  at  some 
distance  below  the  surface ;  these  are  known  locallv  as  kanJcar ; 


154  SOILS  [CH. 

part  of  the  iron  oxide  is  often  also  in  a  nodular  and  concre- 
tionary form. 

Red  soils.  Highly  characteristic  of  the  subtropical  and 
warm  temperate  regions  is  a  group  of  soils  distinguished  by  a 
prevailing  bright  red  tint.  These  are  not  sharply  marked  off 
from  the  laterite  soils  of  the  tropics  on  the  one  hand  or  from  the 
brown  meadow  soils  of  the  cooler  temperate  belt  on  the  other, 
but  they  undoubtedly  form  a  natural  class  of  soils  whose 
characteristics  are  mainly  determined  by  climatic  conditions. 
Red  soils  are  most  perfectly  developed  in  regions  with  a  hot 
summer  and  a  fairly  warm  winter;  they  appear  to  belong  to 
the  northern  part  of  the  arid  belt,  where  a  fair  amount  of  rain 
falls,  at  any  rate  in  the  cooler  part  of  the  year,  though  the 
summer  is  dry.  In  this  they  show  an  affinity  to  the  lateritic 
group.  The  most  favourable  conditions  for  their  development 
are  realized  over  a  great  part  of  the  Mediterranean  region,  in 
parts  of  Spain,  Italy  and  Greece,  in  Corsica,  Sardinia,  the 
Balearic  Islands,  and  over  a  large  tract  in  northern  Africa  and 
Asia  Minor.  A  special  variety  is  the  terra  rossa  of  the  Dalmatian 
coast  of  the  Adriatic1. 

Ordinary  red  soils  show  great  variety  of  constitution, 
ranging  from  sands  to  clays,  and  it  appears  that  the  character- 
istic bright  red  colour  is  conditioned  by  two  circumstances, 
namely,  low  content  of  humus,  and  abundance  of  ferric  oxide; 
these  conditions  are  evidently  determined  mainly  by  climate; 
during  the  mild  and  fairly  moist  winter  humus  is  rapidly 
destroyed,  and  in  the  dry  season  there  is  much  precipitation  of 
iron  oxide  from  the  evaporating  solutions  in  the  soil. 

Ordinary  red  soils  may  apparently  be  formed  under  suitable 
conditions  from  rocks  of  almost  any  kind,  but  terra  rossa  in 
the  strict  sense  is  formed  especially  from  limestones  that 
originally  contained  a  certain  amount  of  iron  and  alumina. 
Terra  rossa  is  best  seen  in  the  Karst  district,  in  the  south-west 
of  the  Austrian  empire,  bordering  the  Adriatic.  This  region 
consists  mainly  of  limestones  and  dolomites  of  Triassic  age, 
and  shows  in  a  high  degree  of  perfection  all  the  special  features 

1  Neumayr,  Verhandl  Geol.  Reichsanstalt,  Vienna,  1875,  p  50.  See  also 
Erdgeschichte,  Vienna,  1895,  p.  453. 


vi]  SOILS  155 

of  limestone  denudation.  Terra  rossa  is  a  very  ferruginous 
clay  that  is  supposed  to  represent  the  insoluble  residue  of  the 
impure  limestones;  it  is  therefore  possibly  comparable  with 
parts  of  the  English  clay-with- flints  (see  p.  114).  However  it 
has  been  suggested  that  the  red  material  is  really  due  to 
precipitation  of  iron  oxide  from  solutions  of  iron  salts  and  not 
directly  to  normal  weathering.  This  question  is  still  undecided. 
It  is  noticeable  that  red  soils  extend  further  north  on  limestones 
and  dolomites  than  on  any  other  sort  of  rock  and  this  fact 
favours  the  "insoluble  residue"  theory;  also  when  specimens 
of  the  underlying  white  limestones  are  dissolved  in  weak  acetic 
acid,  a  small  amount  of  red  ferruginous  clay  is  left  behind, 
hence  the  argillaceous  material  is  really  present  in  the  rock. 

Brown  soils.  Under  this  general  designation  are  here 
included  the  ordinary  arable  and  pasture  soils  of  western  and 
central  Europe,  the  eastern  United  States  and  many  other 
regions  of  cool  temperate  climates.  Even  in  the  areas  thus 
specified  the  annual  range  of  temperature  varies  a  good  deal, 
being  least  in  the  insular  climate  of  the  British  Isles  and  other 
maritime  regions,  increasing  towards  the  interiors  of  the 
continents.  The  chief  determining  factor  seems  to  be  a  fairly 
abundant  but  not  excessive  rainfall,  and  a  range  of  temperature 
that  admits  of  the  free  growth  of  herbaceous  plants,  without 
too  strong  a  tendency  towards  forests  or  peat-formation.  The 
brown  soils  are  in  fact  par  excellence  the  soils  of  fertile  and 
highly  cultivated  lowlands  and  plains,  especially  adapted  for 
arable  land  and  rich  permanent  pasture.  Under  present 
economic  conditions  they  are  mainly  cultivated  on  the  intensive 
system,  with  liberal  manuring  and  heavy  crops  in  regular 
rotation. 

Owing  to  abundant  and  free  water- circulation  much  soluble 
material  is  washed  out  of  the  upper  layers,  especially  sulphates 
and  carbonates,  whereas  alumina  and  phosphoric  acid  tend  to 
accumulate.  In  many  soils  of  this  class  potash  and  lime  tend 
to  be  deficient,  especially  the  former. 

The  colour  of  the  soil  depends  on  a  combination  of  two 
factors:  (1)  the  relative  abundance  and  state  of  oxidation  of 
the  iron,  and  (2)  the  proportion  of  humus.  The  latter  can  be 


156  SOILS  [CH. 

removed  from  a  sample  by  treatment  first  with  very  dilute 
hydrochloric  acid  and  then  with  ammonia,  the  true  colour  of 
the  soil  thus  becoming  apparent.  This  is  found  to  depend 
mainly  on  the  character  of  the  rock  or  other  deposit  from 
which  the  soil  is  derived.  The  colour  is  usually  some  shade 
of  brown,  yellowish  or  reddish,  according  to  circumstances. 
From  red  rocks,  such  as  the  Old  or  New  Red  Sandstone,  red 
soils  are  formed.  When  the  content  of  humus  is  unusually 
high  the  soil  may  be  nearly  black,  while  Chalk  soils  are  often 
very  pale  in  colour.  However  the  calcareous  soils  in  general 
form  a  somewhat  aberrant  group. 

Brown  soils  show  the  utmost  variation  of  agricultural 
character,  ranging  from  the  heaviest  clays  to  light  sands,  and 
include  all  the  varieties  known  as  loams  and  marls.  The 
mineral  composition  and  origin  of  soils  has  already  been  treated 
at  some  length,  and  in  the  later  chapters  of  this  book  a  detailed 
account  is  given  of  the  dominant  agricultural  characters  of  the 
soils  derived  from  the  rock-formations  of  the  British  Isles. 
It  is  therefore  unnecessary  to  pursue  the  subject  further  here. 

Saline  soils.  Under  this  heading  are  included  two  very 
different  types  of  soil,  agreeing  only  in  the  fact  that  both  are 
unusually  rich  in  soluble  mineral  substances.  The  first  group 
includes  the  soils  of  those  areas  which  are  occasionally  over- 
flowed by  the  sea,  or  into  which  sea-water  is  able  to  penetrate 
by  percolation.  They  are  necessarily  confined  strictly  to  coastal 
regions  at  or  near  sea-level.  Such  are  the  salt  marshes  of  the 
Thames  estuary,  of  parts  of  the  Norfolk  coast  and  the  "slob 
lands"  of  Ireland.  The  soil  of  such  areas  is  usually  alluvial  in 
origin,  being  either  sea  or  river  silt  and  commonly  of  a  muddy 
nature.  The  salts  present  are  naturally  those  found  in  sea-water, 
the  most  important  being  sodium  chloride,  with  a  smaller 
proportion  of  magnesium  chloride  and  sulphate.  When 
capable  of  utilization  for  agricultural  purposes  such  areas  are 
always  pasture,  and  the  herbage  is  of  a  peculiar  character, 
though  often  forming  good  feeding  for  stock. 

The  second  type  of  saline  soils  belongs  exclusively  to  the 
arid  climatic  belts  and  occurs  in  those  regions  where  the  drainage 
is  deficient.  In  all  soils  soluble  salts  are  formed  by  chemical 


vi]  SOILS  157 

weathering;  in  moist  climates  these  are  leached  out  by  rain 
and  carried  away  in  solution  in  the  drainage  water,  eventually 
reaching  the  sea,  adding  to  its  stock  of  salt.  But  in  regions 
of  deficient  rainfall  this  leaching  action  is  in  abeyance,  the 
soluble  salts  tending  to  accumulate  in  the  soil,  till  they  may 
become  conspicuous,  as  in  the  salt  deserts  of  south-western 
Asia.  Where  the  drainage  basin  is  entirely  self-contained, 
without  outlet  to  the  sea,  the  effect  of  such  leaching  and  stream- 
action  as  there  may  be  is  to  concentrate  the  salts  in  the  hollows 
of  the  surface.  The  extreme  form  of  this  is  seen  in  the  formation 
of  salt  lakes,  such  as  the  Dead  Sea  and  the  Great  Salt  Lake  of 
Utah,  both  of  which  were  once  fresh,  but  have  now  become 
many  times  salter  than  the  sea  by  concentration  and  evapora- 
tion. In  some  cases  it  has  been  shown  that  the  saltness  of  the 
soil  is  due  to  the  ascent  of  saline  solutions  by  capillary  action 
from  lower  levels,  where  beds  of  soluble  salts  of  an  earlier 
geological  date  have  been  covered  and  preserved  by  later 
sediments.  This  appears  to  be  the  case  with  the  Szek  lands  of 
Hungary  and  some  of  the  salt  soils  of  western  America,  and 
the  extreme  saltness  of  the  Dead  Sea  is  attributed  to  solution 
by  the  Jordan  waters  of  beds  of  rock  salt  and  other  saline 
substances  in  the  Cretaceous  and  Tertiary  strata  of  the  district. 

The  composition  of  the  salts  found  in  such  soils  varies 
widely  according  to  circumstances.  The  commonest  are  mag- 
nesium chloride,  magnesium  sulphate  and  sodium  chloride; 
potassium  compounds  are  less  abundant,  though  almost  always 
present.  Calcium  sulphate  is  also  very  widely  distributed. 
Soils  rich  in  the  constituents  just  mentioned  form  the  true 
saline  soils  in  the  strict  sense,  while  those  characterized  by 
sodium  sulphate  and  sodium  carbonate  are  generally  called 
alkali  soils,  though  some  American  writers  use  the  term  alkali 
soil  indiscriminately  for  both  groups.  German  writers,  on  the 
other  hand,  use  the  term  saline  soils  (Salzboden)  to  include  soils 
with  sodium  sulphate  and  carbonate  as  well  as  those  with 
chlorides. 

Several  different  types  of  saline  soils  have  been  recognized 
by  Glinka  and  others1.  They  are  closely  related  to  the  ordinary 
1  Glinka,  Die  Typen  der  Bodenbildung,  Berlin,  1914,  pp.  177-211. 


158  SOILS  [CH. 

soil-types  of  arid  regions,  being  generally  poor  in  humus  and 
pale  in  colour,  showing  much  resemblance  to  the  bleached  soils 
of  the  German  and  Russian  classifications.  Some  occurrences 
of  this  type  in  hollows  in  the  steppe  regions  are  described  as 
"saline  podzol"  or  as  "solonetz"  (see  p.  151).  Some  varieties 
shrink  and  crack  deeply  when  dry,  developing  a  curious 
columnar  structure,  like  miniature  basaltic  columns.  In  the 
neighbourhood  of  the  Caspian  Sea  there  is  a  considerable  area 
of  white  soil  or  Byelozom,  white  or  grey  in  colour  and  rich  in 
salts.  This  is  characteristic  of  the  southern  dry  steppes,  where 
the  annual  rainfall  is  about  8  inches. 

Alkali  soils.  This  term  is  unfortunately  used  in  a  somewhat 
vague  way,  especially  by  American  writers,  to  include  a  con- 
siderable variety  of  soils  characterized  by  an  excessive  proportion 
of  soluble  salts.  Two  chief  types  are  generally  recognized  in 
America,  namely,  black  alkali  soils,  characterized  by  sodium 
carbonate,  and  white  alkali  soils,  in  which  sodium  sulphate  is 
the  dominant  soluble  constituent.  The  term  "alkali  soil" 
should  strictly  be  reserved  for  the  first  class  only.  Some 
writers  go  even  further  and  include  in  this  group  all  the  soils 
with  a  large  proportion  of  chlorides,  or  what  in  Europe  are 
commonly  designated  saline  soils. 

The  alkali  soils  are,  as  would  be  expected,  restricted  to  the 
arid  regions  of  the  globe ;  they  are  found  extensively  in  western 
America,  northern  Africa,  and  south-western  Asia,  as  well  as 
to  a  more  limited  extent  in  India.  They  probably  occur  also 
in  South  Africa  and  Australia,  though  as  to  this  we  possess 
little  information.  In  Europe  they  occur  in  the  steppe  region 
of  south-east  Russia,  and  to  a  small  extent  in  the  Szek  lands  of 
Hungary,  along  the  course  of  the  river  Theiss.  Both  black 
and  white  alkali  soils  are  represented  here,  although  this  is  not 
an  arid  region,  and  the  salts  may  be  derived  from  underground 
salt  beds1. 

Normally  alkali  soils  are  found  only  in  regions  of  deficient 
rainfall,  where  there  is  no  surface  drainage  to  carry  away  the 
soluble  salts  from  the  soil,  and  the  only  loss  of  water  is  by 

1  Glinka,  Die  Typen  der  Bodenbildung,  Berlin,  1914,  pp.  180-182. 


vi]  SOILS  159 

evaporation.  Under  these  conditions  the  salts  tend  to  accumu- 
late in  the  upper  layers  of  the  soil,  commonly  in  the  uppermost 
three  or  four  feet.  The  nature  of  the  salts  will  of  course  depend 
on  the  source  from  whence  they  are  derived,  that  is,  the  salt- 
content  of  the  underlying  rocks.  Whereas  in  ordinary  saline 
soils  the  chief  salts  are  sodium  and  magnesium  chlorides,  in 
alkali  soils  the  most  important  constituents  are  sodium  sulphate 
(Glauber's  salt)  and  sodium  carbonate.  Soils  rich  in  the  latter 
salt  can  generally  be  distinguished  by  the  occurrences  of  surface 
puddles  of  black  or  dark  brown  water;  hence  the  name  of 
black  alkali  soil  applied  to  this  class.  The  black  colour  is 
supposed  to  be  due  to  humus  dissolved  in  the  salt  solution. 

The  distribution  of  soluble  matter  in  alkali  soils  varies  widely 
according  to  the  meteorological  conditions  prevailing  at  the 
time.  During  rainy  weather  the  salts  are  dissolved  away  from 
the  surface  and  carried  downwards.  When  the  air  is  dry,  on 
the  other  hand,  evaporation  takes  place  from  the  surface  and 
the  salts  accumulate  in  the  upper  layers  of  the  soil.  In  extreme 
cases  an  incrustation  of  salt  crystals  may  even  form  on  the 
surface,  as  in  the  alkali  deserts  of  western  America  and  many 
other  places. 

Alkali  soils  are  usually  very  infertile,  partly  owing  to  the 
deleterious  effect  of  the  strong  salt  solutions  on  plants,  and 
partly  owing  to  the  peculiar  effect  of  alkaline  salts  in  forming 
a  hard  pan  in  the  upper  part  of  the  soil  (see  p.  142).  The 
native  vegetation  is  scanty  and  peculiar,  such  as  the  Australian 
"salt-bush"  and  the  American  "sage-brush,"  most  of  the  plants 
not  being  palatable  to  stock,  while  few  ordinary  agricultural 
crops  will  succeed  on  such  soils.  True  alkali  soils  are  therefore 
generally  barren  and  uncultivated,  though  often  rich  in  the 
elements  of  plant  food,  with  the  important  exception  of  nitrogen, 
which  is  always  scarce.  When  they  can  be  irrigated  sufficiently 
to  wash  the  harmful  salts  out  of  the  soil  they  may  become 
quite  fertile1. 

The  soil-regions  of  India.  The  soils  of  India  have  been 
investigated  by  several  authorities,  especially  by  Voelcker, 

1  Hilgard,  Soils,  New  York. 


160  SOILS  [CH. 

Leather  and  Mann.  Their  general  conclusions  may  be  sum- 
marized as  follows ;  there  are  four  main  types,  distinguished 
by  origin  and  composition;  within  each  type  varieties  can  be 
recognized.  The  four  chief  groups  are: 

(1)  The  alluvium  of  the  Indus,  Ganges  and  Bramaputra. 

(2)  The  black  cotton  soil  or  regur  of  the  Deccan  plateau. 

(3)  The  red  soils  of  Madras. 

(4)  The  laterite  soils. 

(1)  The  alluvium  of  northern  India  varies  greatly  in  its 
character,    according    to    climate    and    rainfall.     In    northern 
India  the  rainfall  is  deficient  in  the  west  and  increases  steadily 
eastwards.     Parts  of  the  Punjab  and  Rajputana  are  very  arid, 
whereas  in  the  Khasia  Hills,  in  the  basin  of  the  Bramaputra, 
is  the  highest  rainfall  of  the  world  (600  inches  per  annum). 
Hence  the  soils  undergo  a  very  varied  amount  of  leaching,  this 
being  the  determining  factor.     The  alluvium  consists  entirely 
of  fine  sand  and  silt,  any  larger  elements  being  rare  or  absent. 
Calcareous   concretions   (kankar)    are   abundant   in   the   drier 
regions.     In  the  west  the  soils  are  of  a  distinctly  arid  type,  and 
in  Aligarh  and  the  Agra  district  even  alkali  soils  (reh  soils)  are 
found.     Over  the  whole  drier  part  of  this  belt,  owing  to  absence 
of  excessive  leaching,  the  soils  are  rich  in  potash,  lime  and 
magnesia.     On    the    other    hand,    in    the    Bramaputra    basin 
leaching  is  excessive  owing  to  the  heavy  rainfall,  and  the  soils 
are  very  poor  in  soluble  matter,  especially  lime.     They  are 
consequently  found  suitable  for  the  growth  of  tea. 

(2)  The  regur  or  black  cotton  soil  forms  a  very  well-marked 
and   distinctive   type,    having   a   wide   distribution   over   the 
plateau  of  the  Central  Provinces,  the  Deccan,  from  the  Vindhya 
mountains  southwards.     The  soil  is  very  deep,  and  possesses  a 
uniform  black  colour,  even  at  a  considerable  distance  from  the 
surface.     Kankar  concretions  are  common.     In  the  dry  season 
the  soil  shrinks  and  forms  very  deep  and  wide  cracks.     In 
regard  to  chemical  composition,  the  percentages  of  potash,  lime 
and   magnesia   are   high,   but  the   nitrogen   is   curiously   low, 
considering  the  notable  fertility  of  the  soil,  which  is  said  to 
have  been  cultivated  continuously  for  over  2000  years  without 


VI] 


SOILS 


161 


manure.  It  can  only  be  supposed  that  nitrification  is  active, 
and  that  a  supply  of  nitrate  sufficient*  for  the  needs  of  the  crop 
is  constantly  being  produced  and  removed  as  fast  as  it  is 
formed.  The  origin  of  this  soil  is  somewhat  of  a  mystery ;  it 
lies  mostly  on  basalt,  but  scarcely  possesses  the  characters  that 
would  be  expected  of  a  sedentary  soil  on  that  rock.  On  the  other 
hand  any  theory  of  formation  by  transport  also  presents 
considerable  difficulties. 

(3)  The  red  soils  of  Madras  are  clearly  of  sedentary  origin 
and  are  mainly  formed  from  metamorphic  rocks,  but  they  are 
also  known  on  other  substrata.  They  are  very  variable  in 
their  chemical  composition,  agreeing  only  in  the  prevailing  red 
tint,  due  to  the  presence  of  finely  disseminated  ferric  oxide. 
The  total  percentage  of  iron  is  not  as  a  rule  notably  high,  and 
the  effectiveness  of  this  substance  as  a  colouring  matter  must 
be  attributed  largely  to  its  fine  state  of  division.  The  red  soils 
are  as  a  rule  less  rich  in  plant  food  and  less  fertile  than  the 
black  cotton  soil.  This  soil  type  is  in  no  way  remarkable  and 
must  be  regarded  merely  as  an  instance  of  the  red  soils  so 
characteristic  of  tropical  and  semi-tropical  regions;  a  general 
description  of  this  type  has  already  been  given. 


Chemical  Composition  of  Indian 
Alluvium 


Soils. 


Alluvium 

of  Bra- 

Regur, 

Laterite 

o 

f  Ganges, 

maputra, 

Trichi 

Red  Soil, 

Soil, 

Punjab 

Bengal 

nopoli 

Madras 

Madras 

Insoluble  matter 

81-57 

84-60 

65-16 

90-47 

76-86 

Potash 

•74 

•35 

•14 

•24 

•0) 

Soda   

•08 

•30 

•01 

•12 

•17 

Lime  

1-44 

•04 

2-18 

•56 

tr. 

Magnesia 

1-97 

•46 

2-47 

•70 

•77 

Ferric  oxide  ... 

4-32 

2-03 

9-27 

3-51 

10-09 

Alumina 

5-85 

5-03 

13-76 

2-92 

8-84 

Phosphoric  acid 

•23 

•05 

tr. 

•09 

tr. 

Carbon  dioxide 

1-13 

— 

•91 

•30 

•12 

Water  and  organic 

2-67 

5-59 

5-85 

1-01 

2-87 

matter 

Total       100-00 

98-50 

99-75 

99-92 

99-81 

(4)    The  laterite  soils  occur  chiefly  in  the  coast  region  of 
Bengal,  in  Madras  and  on  the  Malabar  coast.     They  appear  to 


R.  A.  G. 


11 


162  SOILS  [CH. 

grade  into  the  red  soils  in  places,  and  need  no  further  description 
here.  (For  a  general  account  of  such  soils  see  p.  154,  and  for 
the  origin  of  laterite  p.  115.) 

The  table  on  p.  161  shows  analyses  of  a  typical  example  of 
each  of  the  types  mentioned  above. 

General  classification  of  climatic  soil-zones.  From  a  study 
of  the  work  on  the  geographical  distribution  of  soil-types 
carried  out  by  many  investigators  in  Germany,  Russia  and 
America,  certain  general  conclusions  can  be  drawn.  In  the 
first  place  it  is  clear  that  in  soil-formation  the  dominant  con- 
trolling factor  is  climatic  rather  than  geological ;  that  is  to  say, 
that  under  determinate  conditions  of  temperature  and  rainfall, 
rocks  and  superficial  deposits  of  the  most  varied  kind  may  give 
rise  to  almost  identical  soil-types.  Of  these  climatic  conditions 
the  most  potent  is  undoubtedly  the  rainfall ;  temperature  plays 
a  secondary  part,  and  its  action  is  indirect  rather  than  direct ; 
temperature  is  certainly  of  great  importance  in  controlling  the 
growth  of  vegetation,  both  quantitatively  and  qualitatively. 
It  is  largely  through  vegetation  (or  the  want  of  it)  that  the 
special  characters  of  each  soil  are  developed. 

So  far  as  the  rainfall  is  concerned,  its  absolute  amount  is  the 
chief  consideration,  but  it  must  not  be  forgotten  that  the 
relative  moisture-content  of  the  atmosphere  also  has  much 
influence;  in  a  very  hot  region  much  more  water  vapour  is 
required  to  saturate  the  atmosphere  than  in  a  cold  one,  hence 
with  different  annual  average  temperatures  the  same  rainfall 
may  produce  dry  air  in  one  case  (that  is,  air  far  below  its 
saturation  point),  and  in  the  other  case  air  completely  saturated 
with  moisture.  The  effect  on  vegetation  will  obviously  be 
very  variable ;  one  region  may  be  inhabited  by  a  swamp-flora, 
the  other  by  xerophytic  plants. 

Even,  however,  when  this  cause  of  variation  has  been 
brought  into  account,  it  appears  that  the  total  rainfall  is  the 
most  important  controlling  influence  in  soil-formation.  In  a 
dry  region  chemical  decomposition  is  in  abeyance;  rock- 
disintegration  is  mainly  mechanical,  and  wind- transported 
deposits  are  the  rule.  Increased  rainfall  gives  rise  to  more 
active  chemical  and  bacterial  processes,  and  water-transport  is 


vi]  SOILS  163 

dominant.  In  high  northern  latitudes  the  conditions  are 
special,  owing  to  the  more  or  less  permanent  freezing  of  the 
subsoil;  this  leads  to  the  development  of  the  tundra  type, 
which  is  independent  of  rainfall. 

On  a  climatic  basis  the  earth  can  be  divided  into  soil- zones, 
which  are  of  general  application,  as  follows1: 

Annual  rainfall  Soil  type 

0-  8  inches  ...         ...  Desert. 

8-16       „  ...       '-.;...  £    Steppe. 

16-24       „  Prairie. 

over  24    „  ...         ...  Forest. 

Arctic  temperature Tundra. 

These  zones  are  only  applicable  with  strictness  to  regions 
of  the  world,  such  as  Russia,  North  and  South  America  and 
East  Africa,  which  are  still  in  a  more  or  less  natural  state. 
In  western  Europe,  India,  China  and  other  lands  of  ancient 
civilization  there  has  been  so  much  interference  by  the  hand 
of  man,  through  clearing  of  forests,  arable  cultivation,  irrigation 
and  other  processes,  that  the  original  character  of  the  soil  has 
been  profoundly  modified.  It  appears  however  that  the  greater 
part  of  the  British  Isles,  France  and  southern  and  western 
Germany  originally  belonged  to  the  forest  type,  as  also  does  the 
eastern  part  of  the  United  States  and  of  Canada.  Tropical 
forests  are  chiefly  found  in  South  America,  Central  Africa  and 
parts  of  southern  Asia  and  the  adjacent  islands.  As  European 
examples  of  the  prairie  type  we  may  instance  Hungary  and 
central  Russia ;  here  also  must  be  classed  the  prairies  of  North 
America,  the  Savannas  and  Llanos  of  South  America,  the 
grassy  park-like  plains  of  East  Africa,  and  the  pastoral  districts 
of  Australia.  Steppe  conditions  are  found  largely  developed 
in  Russia  and  Central  Asia,  Spain,  South  Africa,  the  south- 
western United  States  and  parts  of  Australia.  It  is  unnecessary 
here  to  enumerate  the  desert  regions  of  the  world,  which  are 
widely  spread  both  in  the  northern  and  southern  hemispheres ; 
they  are  of  no  agricultural  importance,  though  of  great  interest 
to  the  geologist.  It  is  possible,  however,  that  in  the  future, 

1  Von  Cholnoky,  "Uber  die  fur  Klimazonen  bezeichnenden  Bodenarten," 
Comptes  Rendus  l*re  Conf,  Internal.  Agrogeol.  Budapest,  1909,  pp.  163-176. 

11—2 


164  SOILS  [CH, 

by  means  of  irrigation,  large  areas  now  desert  may  be  brought 
under  profitable  cultivation. 

Soil  types  in  Britain.  When  we  come  to  consider  in  detail 
the  soils  of  any  particular  area  we  shall  find  wide  apparent 
variations  in  their  characters,  especially  in  regions  of  such 
variable  geological  structure  as  the  British  Isles.  That  this  is- 
the  case  will  appear  from  the  chapters  devoted  to  the  con- 
sideration of  the  stratified  and  other  rocks  of  this  country 
(Chapters  ix  to  xvi).  Nevertheless .  when  climatic  factors 
are  also  taken  into  account,  it  appears  possible  to  distinguish 
some  broadly  defined  regions  in  which  certain  similarities  run 
through  soils  of  the  most  varied  origins.  These  broad  variations 
depend  largely  on  temperature,  rainfall,  relief  of  the  ground  and 
vegetation,  the  last  being  largely  controlled  by  the  other  three. 
By  a  careful  comparison  of  British  soil  types  with  those  pre- 
viously discussed  as  characteristic  of  certain  climatic  zones  in 
Europe  and  America  it  will  doubtless  in  time  become  possible 
to  assign  each  region  to  its  proper  class.  As  yet  sufficient 
materials  are  not  available  for  detailed  statements  of  this  kind, 
but  even  now  a  small  attempt  may  be  made  on  general  lines. 
The  differences  mainly  depend  on  the  extent  to  which  soluble 
salts  have  been  washed  out  of  the  soil  by  percolating  water, 
which  is  evidently  a  matter  of  rainfall.  As  would  naturally 
be  expected  there  are  marked  differences  in  this  respect  between 
the  eastern  and  the  western  sides  of  the  country,  the  soils  on 
the  west  being  more  washed  than  those  on  the  east.  In  Devon 
and  Cornwall  and  in  the  south-west  of  Ireland  this  effect  of 
rainfall  is  partly  compensated  by  the  higher  temperature, 
which  causes  a  greater  luxuriance  of  plant  growth.  In  the 
Highlands  of  Scotland  and  to  a  less  extent  in  the  north-west  of 
Ireland,  lower  temperature  and  greater  elevation  are  specially 
favourable  to  the  growth  of  peat.  Many  of  the  soils  here 
consist  almost  entirely  of  sand  and  humus,  all  soluble  matter 
having  been  washed  away.  In  the  western  half  of  England 
the  loss  of  soluble  matter  is  greater  than  in  the  east,  and  the 
soils  are  often  deficient  in  some  of  the  elements  of  plant  food. 
In  the  dry  regions  of  the  east  soluble  plant  foods  tend  to  con- 
centrate in  the  soil  owing  to  the  smaller  amount  of  percolating 


vi]  SOILS  165 

water,  while  humus  is  on  the  whole  less  abundant.  (Here  such 
exceptional  cases  as  the  Fenland,  due  to  special  causes,  must 
be  eliminated.)  On  the  whole  the  soils  of  the  west  of  the 
British  Isles  seem  to  approximate  to  the  Podzol  type  of 
Russian  authors,  while  those  of  the  eastern  parts  may  be 
regarded  as  typical  "brown  soils"  of  the  continental  classifica- 
tion. Nevertheless  there  are  many  local  variations  and  the 
above  statement  must  be  regarded  as  merely  an  approximation 
to  the  truth.  The  detailed  study  of  the  soils  of  an  area  of 
sufficiently  varied  structure  and  relief  would  certainly  lead  to 
interesting  results  from  this  point  of  view;  it  may  perhaps 
be  suggested  that  Yorkshire  offers  a  promising  field  for  in- 
vestigation. The  four  chief  regions,  the  western  hills,  the 
central  plain,  the  Cleveland  Hills  and  the  Wolds,  vary  so 
much  in  altitude,  climate  and  rock  constitution  that  many 
of  the  chief  soil  types  of  continental  authors  should  be  here 
recognizable.  At  any  rate,  when  regarded  from  the  conven- 
tional standpoint,  the  county  seems  to  contain  soils  of  almost 
every  conceivable  kind. 


CHAPTER   VII 

THE  GEOLOGY  OF  WATER  SUPPLY  AND  DRAINAGE 

One  of  the  most  important  agricultural  applications  of 
geology  is  in  connexion  with  water  supply.  It  is  seldom  that 
a  farm  has  the  advantage  of  access  to  a  public  water  supply; 
even  if  such  is  available  for  the  house  and  buildings,  the  live 
stock  in  the  fields  in  general  have  to  depend  on  ponds,  springs, 
wells  and  streams ;  in  other  words  on  the  natural  water  supply 
of  the  land.  Every  field  that  is  likely  to  be  pastured  by  stock 
should  have  a  sufficient  supply  of  pure  water,  preferably 
running  water,  but  seldom  is  this  state  of  things  attained  or 
even  possible  of  attainment. 

Water  supply  is  entirely  a  geological  question ;  the  primary 
source  of  all  is  of  course  rainfall,  and  in  this  sense  it  is  a  matter 
of  climate;  but  given  a  sufficient  rainfall,  the  availability  of 
this  for  practical  purposes  depends  on  the  nature  and  distribution 
of  the  rocks  of  the  district,  together  with  the  superficial  deposits 
that  in  most  places  overlie  the  hard  rocks,  these  being  often  of 
great  importance  as  controlling  factors. 

Rainfall.  As  is  well  known  to  every  one,  rainfall  varies 
much  from  place  to  place.  In  some  regions,  such  as  the  coast 
of  Peru,  parts  of  the  Sahara  and  the  interior  of  Australia,  there 
is  none  at  all.  Over  wide  areas  of  the  world  the  climate  may  be 
described  as  dry  or  arid,  leading  to  the  formation  of  deserts. 
In  these  the  annual  rainfall  is  less  than  10  inches.  When  the 
rainfall  exceeds  this  figure,  or  even  in  favourable  localities  when 
it  falls  considerably  below  this  limit,  some  sort  of  agriculture 
is  possible,  especially  the  keeping  of  sheep  and  goats.  Here, 
of  course,  the  possibility  of  artificial  irrigation  is  for  the  present 
left  out  of  account.  Over  the  greater  part  of  the  temperate 


CH.  vn]       WATER  SUPPLY  AND  DRAINAGE  167 

zone  the  annual  rainfall  exceeds  20  inches,  while  in  parts  it  is 
far  higher  than  this.  Even  within  the  limits  of  the  British 
Isles  there  is  great  variation.  In  the  south-eastern  counties  the 
average  rainfall  is  about  24  inches ;  to  the  north  and  west  of  a 
line  drawn  from  the  mouth  of  the  Tees  to  Bristol  it  nearly 
always  exceeds  30  inches,  while  at  special  places  it  is  far  greater ; 
at  Seathwaite  in  Cumberland  the  annual  average  is  about 
130  inches,  while  here,  as  also  near  Snowdon  and  at  Ben  Nevis, 
records  of  220  to  230  inches  per  annum  have  been  obtained. 
The  highest  rainfall  of  the  world,  so  far  as  known,  is  found  in 
the  Khasia  Hills  in  northern  Bengal,  in  the  foothills  of  the 
Himalayas,  where  the  annual  average  is  about  600  inches, 
nearly  all  of  which  falls  in  about  three  months.  This  is  of  course 
due  to  exceptional  conditions,  namely,  the  occurrence  of  a 
regular  monsoon,  with  well-marked  wet  and  dry  seasons. 

Of  the  total  amount  of  water  that  falls  as  rain,  it  has  been 
estimated  that  one-third  runs  off  at  once  as  streams,  one-third 
sinks  into  the  ground  to  form  underground  water  and  one-third 
passes  back  into  the  air  by  evaporation.  It  is  with  the  first 
two  portions  that  we  are  here  concerned.  It  is  unnecessary 
however  to  say  much  about  surface  streams;  the  origin  and 
development  of  river  systems  has  already  been  described  in 
Chapter  in,  and  it  is  obvious  that  a  river  or  stream  can  be 
utilized  in  many  ways  for  agricultural  purposes.  Live  stock 
can  obtain  their  drinking  water  directly  from  the  river ;  water 
can  be  pumped  from  the  river  into  artificial  ponds  and  ditches, 
and  thus  conducted  from  place  to  place;  under  favourable 
conditions  water  power  can  be  used  to  drive  agricultural 
machinery  and  so  on.  Besides  this,  there  is  the  very  important 
subject  of  irrigation.  In  Britain  this  process  is  not  much 
employed,  being  confined  chiefly  to  the  water  meadows  of  the 
south-western  counties  and  to  sewage  farms.  In  dry  regions 
however,  such  as  South  Africa,  Australia  and  parts  of  western 
America,  irrigation  is  of  enormous  and  increasing  importance. 
This,  however,  is  engineering  rather  than  geology  and  the 
subject  cannot  here  be  pursued  further. 

Underground  water.  The  geology  of  water  supply  deals 
mainly  with  the  second  fraction  as  above  described,  namely, 


168  THE   GEOLOGY  OF  [CH. 

the  water  that  sinks  into  the  ground.  The  ultimate  fate  of  this 
must  obviously  depend  on  the  geological  structure  of  the  area 
and  on  the  nature  of  the  rocks  composing  it.  The  most 
important  consideration  here  is  evidently  the  permeability  or 
otherwise  of  the  rocks.  If  they  are  totally  impermeable, 
water  cannot  sink  into  the  ground  at  all;  it  must  run  off  as 
streams  or  be  evaporated  into  the  atmosphere,  or  it  may,  of 
course,  remain  on  the  surface  as  ponds,  lakes  and  swamps,  if 
the  configuration  is  favourable.  Ordinarily  the  rocks  are 
more  or  less  permeable  and  the  water  works  downwards, 
forming  what  is  known  as  ground  water.  Experience  in  deep 
mines  shows,  however,  that  this  water  has  not  an  indefinite 
downward  extension;  below  a  certain  depth,  depending  on 
local  conditions,  all  mines  are  dry.  There  is  one  exception  to 
this  rule ;  in  certain  volcanic  districts  hot  water  comes  up 
in  large  quantity  from  a  great  depth  within  the  crust,  in  the 
form  of  geysers  and  hot  springs.  This  water  is  evidently  of 
different  origin  from  the  atmospheric  water  previously  considered 
and  is  believed  to  be  derived  directly  from  the  heated  igneous 
material  of  the  interior  of  the  earth.  It  is  of  no  agricultural 
importance  and  need  not  be  considered  further. 

Under  ordinary  circumstances,  then,  a  certain  zone  of  the 
earth's  crust  is  saturated  with  water  of  atmospheric  origin, 
which  is  often  called  meteoric  water ;  the  lower  limit  of  this  is 
indefinite,  but  the  upper  limit  is  much  better  defined  and  is 
often  called  the  water  table.  This  surface  however  is  not  by 
any  means  horizontal,  and  in  a  general  way  it  often  follows  the 
undulations  of  the  land,  maintaining  a  sort  of  rough  parallelism. 
In  places  the  water  table  intersects  the  land  surface  and  gives 
rise  to  springs,  which  often  tend  to  occur  in  lines.  The  position 
of  the  water  table  is  very  largely  controlled  by  the  degree  of 
porosity  of  the  rocks  and  their  capacity  on  the  one  hand  for 
holding  water  or  on  the  other  hand  for  preventing  its  passage. 

Pervious  and  impervious  rocks.  The  amount  of  water  that 
can  be  contained  by  any  given  rock,  when  saturated,  depends 
on  its  texture,  that  is  to  say  chiefly  on  the  size  of  the  particles 
and  the  amount  of  open  space  between  them.  Loose  and  inco- 
herent deposits,  like  gravel  and  sand,  will  hold  more  water  than 


VII] 


WATER  SUPPLY  AND  DRAINAGE 


169 


similar  rocks  that  have  been  welded  into  a  solid  mass  by 
deposition  of  some  cementing  material  in  the  pores  between  the 
particles.  Another  consideration  of  very  great  importance  is 
the  presence  or  otherwise  in  the  rocks  of  open  joints,  which 
may  facilitate  the  passage  of  water  through  even  the  most 
naturally  impervious  rocks,  and  to  a  certain  extent  may  even 
permit  storage  in  them.  Even  the  most  impervious  rocks  will 
absorb  and  hold  a  certain  amount  of  water,  but  if  the  grain 
of  the  rock  is  very  fine,  internal  friction  will  prevent  the  move- 
ment of  this  water.  On  the  other  hand  water  flows  freely 
through  rocks  with  plenty  of  pore-space,  where  the  internal 
friction  is  small. 

The  following  table  gives  a  classification  of  the  common 
rock-types  according  to  their  behaviour  towards  ground  water, 
under  three  headings: 


I.    Porous  and 

II.    Rocks  holding 

III.   Imper- 

permeable rocks 

water  in  fissures 

vious 

rocks 

Sand 

Quartzite 

Clay 

Sandstone 

Grit 

Shale 

Gravel 

Conglomerate 

Marl 

Sandy  limestone 

Marble 

Brickearth 

Chalk 

Slate 

Marble 

Ik  n 

Oolite 

Granite 

Schist 

wnen 

Dolomite 

Greenstone 

Granite 

not 

Brown  ironstone 

Gneiss 

Greenstone 

fissurec 

Schist 

Springs.  When  rain  falls  on  a  surface  of  porous  soil  or  rock 
it  sinks  in  and  works  its  way  downwards  until  it  comes  to  some 
obstruction,  commonly  a  mass  of  impervious  rock.  Its  further 
course  will  depend  on  the  form  and  disposition  of  this  barrier. 


•  ••••    •-.  <^v^>^  S 


Fig.  34.  Formation  of  springs  in  a  hill  of  horizontal  strata,  pervious  above, 
impervious  below.  The  broken  line  indicates  the  upper  limit  of  saturation 
(water  table).  S,  S= springs. 

The  simplest  case  is  where  a  hill  consists  of  horizontal  strata, 
pervious   above,   impervious   below,    as   shown   in   the   figure. 


170 


THE   GEOLOGY   OF 


[CH, 


Then  the  water  will  come  out  as  a  line  of  springs  or  of  ill-defined 
soakage,  along  the  junction  of  the  two  rocks;  in  this  case  all 
round  the  hill.  If  the  strata  are  inclined  the  water  will  of  course 
tend  to  run  more  freely  down  the  dip,  but  owing  to  internal 


Fig.  35.     Formation  of  springs  in  a  hill  of  inclined  strata. 
S  =  principal  spring.  s  =  subsidiary  spring. 

friction  there  may  be  springs  on  the  upper  side  also.  Theoreti- 
cally the  water  might  issue  at  any  point  along  the  plane  of 
contact,  but  in  practice,  owing  to  slight  local  inequalities  which 
tend  to  become  accentuated  in  course  of  time,  there  are  usually 
more  or  less  well  defined  springs  at  certain  points.  These 
form  streams  and  cut  gullies  on  the  hills,  thus  helping  to  keep 
the  springs  to  a  fixed  point  of  discharge.  There  are  many 
possible  arrangements  of  rocks  that  can  give  rise  to  springs,, 
but  the  general  principle  is  the  same  in  all  cases;  the  water 
percolates  through  permeable  rocks  and  is  stored  in  them,, 
until  the  rock  can  hold  no  more,  when  it  overflows  as  a  spring. 
Owing  to  special  local  causes  springs  may  be  thrown  out  at 
various  points.  Dykes  of  igneous  rock  cutting  across  the 


Fig.  36.  A  vertical  dyke  of  impervious  igneous  rock,  cutting  across  inclined 
strata,  causes  a  spring  at  S  by  holding  up  the  water  in  the  pervious  (dotted) 
stratum  to  the  left. 

strata  are  generally  impervious  and  may  originate  an  accumu- 
lation of  water,  with  outflow  of  a  spring,  in  the  middle  of  a 
permeable  series  (see  Fig.  36).  Again  faulting  may  give  rise 
to  favourable  conditions,  by  bringing  impervious  strata  against 
permeable  ones,  and  so  forming  a  basin.  The  conditions  here 


VIl] 


WATEK  SUPPLY  AND  DKAINAGE 


171 


may  somewhat  resemble  those  necessary  for  an  artesian  welly 
to  be  presently  described.  In  point  of  fact  the  possible  varia- 
tions are  endless,  and  no  good  purpose  would  be  served  by  a 
further  detailed  consideration  of  them. 


Fig.  37.  Spring  due  to  faulting  of  inclined  strata.  The  permeable  (dotted) 
bed  is  brought  by  the  fault  F  against  an  impervious  bed,  and  water  is 
held  up  in  it,  till  it  rises  to  the  point  S,  where  it  overflows  as  a  spring. 

Wells.  The  essential  feature  of  a  well  is  that  it  must  cut 
the  water  table  or  upper  limit  of  saturation.  If  this  condition 
is  fulfilled  the  portion  of  the  well  below  this  level  will  be  per- 
manently filled  with  water.  The  conditions  may  even  be  such 
that  the  water  rises  to  the  surface,  or  even  above  it  as  a  jet. 
Such  are  called  flowing  or  artesian  wells.  More  commonly 
however  the  water  has  to  be  brought  to  the  surface  by  a  bucket 
and  rope,  or  by  a  pump.  If  the  level  of  the  water  is  not  more 
than  about  30  feet  from  the  surface  the  common  suction  pump 
may  be  used ;  beyond  this  depth  it  must  either  be  brought  up 
laboriously  by  dipping  and  winding,  or  by  some  sort  of  force 
pump,  or  ram.  For  this  purpose  windmills  are  very  commonly 
employed,  being  both  cheap  and  effective. 

In  choosing  the  position  for  a  well  it  is  essential  to  pay 
attention  to  geological  structure.  A  shallow  well  sunk  in  a 
porous  superficial  stratum,  such  as  gravel,  will  generally  yield 
a  supply  of  water  which  percolates  from  the  surface.  Such 
water  is  specially  liable  to  contamination  and  is  unsuited  to 
domestic  use,  or  for  dairy  purposes,  though  it  may  be  employed 
in  stables  or  for  field  troughs.  The  danger  is  somewhat 


172 


THE   GEOLOGY  OF 


[CH. 


decreased  if  the  well  is  lined  with  water-tight  cemented  brick- 
work to  as  great  a  depth  as  possible,  so  that  water  can  only 
enter  after  being  filtered  through  a  considerable  depth  of 
gravel  or  sand.  In  all  cases  special  care  should  be  taken  that 
surface  water  does  not  run  directly  into  a  well  used  for  domestic 
supply.  This  can  be  guarded  against  by  a  raised  rim. 

The  most  satisfactory  wells  are  undoubtedly  those  in  which 
the  water  has  to  filter  itself  by  passing  through  a  considerable 
thickness  of  porous  strata  in  some  such  way  as  is  shown  in 


W 


Fig.  38.  Conditions  favourable  for  a  well  (W).  The  water-bearing  stratum 
(dotted)  lies  between  two  impervious  beds.  The  rain  falling  on  the  higher 
ground  to  the  left  runs  down  the  dip  and  is  tapped  by  the  well;  it  should 
rise  to  a  considerable  height  above  the  top  of  the  dotted  bed,  possibly 
even  to  the  surface. 

Fig.  38.  Here  the  risk  of  contamination  is  small,  the  water 
being  derived  from  an  upland  region,  and  taken  at  a  lower 
level  after  percolating  through  a  good  thickness  of  sand.  Tf 
however  there  are  buildings  situated  on  the  outcrop  of  the  sand 
(Fig.  39)  there  may  be  danger  even  in  this  instance. 


Fig.  39.  Diagram  to  show  how  the  water  of  a  well  may  be  in  danger  of  pollu- 
tion from  a  village  lying  on  the  water-bearing  stratum  further  up  the 
dip-slope.  W  =  well  sunk  through  impervious  rock  to  pervious  stratum. 

The  best  of  all  water  supply  is  that  obtained  from  deep 
artesian  borings,  such  as  those  that  penetrate  into  the  Chalk 
in  various  parts  of  London.  The  general  principle  involved  is 
shown  in  Fig.  40.  Here  a  pervious  stratum  lies  between  two 
beds  of  clay,  the  whole  series  being  bent  into  a  trough  or  syncline. 
Rain  falls  on  the  outcrop  of  the  pervious  bed  and  runs  down 
the  dip  towards  the  centre  of  the  trough;  here  it  is  under 


vn]  WATER  SUPPLY  AND   DRAINAGE  173 

pressure  of  a  considerable  head  of  water,  owing  to  the  difference 
of  level,  and  if  a  boring  is  made  through  the  upper  bed  of  clay, 
the  water  will  rise  in  it  and  may  form  a  jet  rising  above  the 
surface.  This  happened  when  the  first  deep  borings  were  made 
in  the  London  basin,  but  now  owing  to  the  number  of  such  deep 
wells,  the  pressure  is  much  reduced  and  the  water  generally  has 


Fig.  40.  Conditions  for  artesian  well.  A  water-bearing  stratum  lies  between 
two  impervious  ones,  the  whole  being  folded  into  a  trough  or  syncline. 
The  water  falling  on  the  hills  at  either  end  runs  down  the  dip  and  collects 
at  the  bottom;  when  the  upper  bed  is  pierced  by  the  well  the  water  is 
forced  up  by  the  pressure  of  water  above  and  may  form  a  jet  above  the 
surface. 

to  be  pumped  to  the  surface  from  some  depth.  As  shown  in 
Fig.  37,  a  somewhat  similar  effect  may  be  produced  by  a  fault, 
bringing  a  thick  bed  of  impervious  rock  against  a  pervious 
inclined  stratum.  The  name  artesian  well  is  derived  from  the 
province  of  Artois  in  the  north  of  France,  where  they  were 
first  employed. 

Drainage.  A  large  proportion  of  the  agricultural  soils  of 
the  British  Isles  contain  normally  more  water  than  is  beneficial 
for  crops;  the  object  of  land  drainage  is  to  ameliorate  this 
condition  of  the  soil  and  to  promote  the  growth  of  crops  by 
getting  rid  of  the  hurtful  excess  of  water.  Certain  large  tracts 
of  land  are  water-logged  simply  by  reason  of  their  low  elevation ; 
whatever  the  nature  of  the  soil  and  of  the  underlying  rocks  the 
water  cannot  run  off,  if  there  is  no  fall,  as  for  instance  in  the 
Fenland  of  eastern  England  and  some  of  the  "moors"  of 
Somerset.  Here  pumping  is  obviously  essential,  as  well  as 
drainage,  and  the  whole  matter  becomes  one  of  engineering 
on  a  large  scale.  In  this  place  however  attention  must  be 
confined  to  the  geological  aspect  of  ordinary  farm  drainage 
where  sufficient  fall  is  available  to  carry  off  the  water,  if  it 
can  be  got  into  pipes  or  open  culverts. 

Rocks  have  already  been  classified  into  pervious  and 
impervious:  as  a  first  general  rule  it  may  be  laid  down  that 


174  THE   GEOLOGY  OF  [CH. 

soils  lying  on  and  derived  from  the  impervious  rocks  require 
artificial  drainage,  while  the  others  do  not.  There  are,  of  course, 
exceptions  to  both  these  statements,  due  to  particular  causes, 
but  the  general  principle  holds  good.  The  presence  or  absence 
of  open  joints  in  the  rocks  below  is  evidently  an  important 
factor.  Many  high-lying  and  moorland  areas,  though  lying  on 
light  soils,  are  actually  in  need  of  drainage  from  a  combination 
of  causes ;  in  the  first  place  the  cool  damp  climate  favours  the 
formation  of  peat  (see  p.  118),  and  this  in  itself  indicates  a 
water-logged  condition  of  the  soil.  Again  in  such  places 
impervious  "pans"  are  often  formed  (see  p.  140),  thus  holding 
water  in  the  upper  layers  of  the  soil.  If  the  pan  can  be  broken 
tip  by  mechanical  means,  e.g.  steam  ploughing,  the  necessity 
for  pipe-draining  may  be  obviated. 

The  simplest  type  of  drainage,  often  employed  in  upland 
regions,  is  that  called  "sheep  draining"  ;  this  consists  in  cutting 
a  network  of  surface  channels  or  shallow  gutters  in  suitable 
directions  to  carry  off  the  rain-water  before  it  can  sink  into  the 
ground.  This  can  evidently  only  be  effective  on  sloping 
ground,  where  the  run-off  will  be  fairly  quick,  and  the  channels 
must  cross  the  slopes  obliquely.  Surface  drains  and  open 
drains  of  all  kinds  can  only  be  makeshifts  in  agricultural  lands, 
though  deep  open  drains  may  sometimes  be  profitably  employed 
in  plantations  of  timber  trees,  where  the  existence  of  deep 
holes  will  do  no  harm  to  man  or  beast.  In  nearly  all  cases 
where  draining  is  necessary  at  all,  it  must  be  effected  by  means 
of  pipes,  or  tiles  as  they  are  often  called  technically. 

It  is  usually  considered  that  to  render  a  soil  capable  of 
growing  its  best  crops,  the  upper  surface  of  the  ground  water 
(water  table)  should  be  kept  at  least  4  feet  below  the  surface. 
This  does  not  necessarily  mean  however  that  the  tiles  must 
always  be  at  that  depth.  Owing  to  capillarity  water  will  rise 
into  drains  for  some  distance  above  the  water  surface.  The 
drainage  of  a  sloping  field  of  uniform  deep  soil  is  a  simple  matter, 
the  depth  and  distance  apart  of  the  tiles  depending  chiefly  on 
the  texture  of  the  soil.  Particulars  of  this  nature  will  be  found 
in  any  treatise  on  agriculture  and  need  not  be  dealt  with  here, 
since  they  are  scarcely  geological. 


vii]  WATER  SUPPLY  AND  DRAINAGE  175 

Difficult  problems  in  drainage  sometimes  arise  when  the 
conditions  all  over  the  area  dealt  with  are  not  uniform.  For 
example,  in  low-lying  ground  it  is  sometimes  observed  that  some 
patches  are  wet  and  in  need  of  drainage,  while  other  parts  of 
the  same  field  may  be  sufficiently  dry.  This  generally  arises 
from  the  occurrence  below  the  soil  of  an  uneven  layer  of  imper- 
vious material,  such  as  boulder-clay.  The  worst  instances 
occur  where  the  land  includes  the  sites  of  ponds  or  meres, 
now  dried  up  and  filled  with  alluvium.  The  former  existence 
of  a  pond  here  implies  defective  drainage,  a  condition  unlikely 
to  be  ameliorated  by  natural  processes.  Similar  effects  may 
be  brought  about  by  local  development  of  pans  (see  p.  140). 
In  such  places  every  drainage  scheme  has  to  be  contrived  to 
suit  the  local  conditions  and  each  dried-up  pond  must  generally 
be  dealt  with  separately.  The  former  existence  of  these  can 
often  be  demonstrated  by  the  occurrence  on  their  sites  of 
masses  of  fresh- water  shells;  of  such  a  nature  are  the  well- 
known  deposits  of  white  shell-marl  that  mark  the  sites  of  the 
former  "meres"  of  the  Fenland.  Occasionally  it  so  happens 
that  such  areas  can  be  drained  by  the  simple  process  of  digging 
a  hole,  called  a  dumb- well,  deep  enough  to  penetrate  to  a 
pervious  stratum  below.  This  process  can  also  sometimes  be 
employed  successfully  to  get  rid  of  small  surface  streams  on 
heavy  clay  land ;  if  the  clay  is  not  thick  it  may  be  possible  to 
construct  an  artificial  "swallow  hole"  down  which  the  stream 
can  flow.  It  is  necessary  however  to  be  very  careful  not  to 
divert  polluted  water  into  a  water  supply  system  by  this 
method. 

Ponds.  A  pond  is  essentially  a  hollow  in  the  ground  with 
an  impervious  bottom  and  sides,  intended  for  storage  of  water. 
The  simplest  kind  of  pond  is  one  excavated  in  a  bed  of  some 
impervious  material,  such  as  clay ;  here  no  further  preparation 
is  required  beyond  the  making  of  the  excavation — the  pond  will 
hold  water  naturally,  and  will  maintain  itself  so  long  as  the 
supply  of  water  from  rain  or  from  springs  is  at  least  equal  to 
the  natural  evaporation  and  the  artificial  consumption.  But 
if  the  excavation  is  to  be  made  in  porous  material  some  means 
must  be  adopted  to  render  the  pond  watertight.  This  is  usually 


176  THE   GEOLOGY  OF  [CH. 

effected  by  the  process  known  as  puddling  with  clay,  or  by 
making  some  sort  of  stone  pavement.  For  puddling  any  kind 
of  stiff  heavy  clay  will  be  found  effective ;  if  it  is  free  from  hard 
lumps  or  stones,  so  much  the  better ;  stony  boulder- clay  should 
if  possible  be  avoided,  since  the  stones  are  apt  to  work  out  and 
leave  holes  in  the  floor.  If  the  pond  is  to  be  floored  with  stones 
or  flags  these  should  be  thick  and  heavy  in  order  to  avoid 
displacement  from  the  trampling  of  live  stock.  The  joints 
between  them  can  usually  be  made  sufficiently  water-tight  by 
ramming  with  clay.  Cemented  ponds  are  as  a  rule  only  con- 
structed at  the  homestead  or  near  buildings,  since  they  are 
expensive  to  make. 

The  water  supply  of  ponds  may  be  derived  from  various 
sources;  if  the  excavation  is  made  deep  enough  to  intersect 
the  water  table,  the  supply  will  be  dependent  on  the  level  of 
the  latter.  In  most  places  this  is  fairly  constant,  though 
usually  rising  somewhat  in  winter  and  sinking  in  summer.  If 
the  pond  is  not  deep  enough  it  will  dry  up  in  summer.  Where 
it  is  not  possible  to  excavate  down  to  the  permanent  water- 
level,  the  supply  from  rainfall  may  be  sufficient  to  last  the  whole 
year,  especially  if  surface  water  from  surrounding  slopes  runs 
in  freely.  Again,  it  is  very  often  possible  to  utilize  some  small 
surface  stream,  however  insignificant,  by  means  of  an  open 
ditch  or  pipes,  or  even  the  drainage  water  from  some  high-lying 
tract  may  be  led  into  the  pond.  The  construction  of  reservoirs 
on  any  considerable  scale  does  not  usually  come  within  the 
scope  of  agricultural  practice,  though  it  is  obvious  that  it  is 
impossible  to  draw  any  hard  and  fast  line  of  distinction  between 
a  pond  and  a  reservoir.  If  it  is  desired  to  construct  a  dam 
across  a  stream,  care  must  be  taken  to  see  that  the  foundation 
is  suitable.  A  dam  of  any  size  can  only  be  constructed  with 
success  on  a  foundation  of  some  hard  rock,  though  smaller 
dams  may  be  founded  on  a  stiff,  tough  clay;  sand,  gravel  or 
alluvial  deposits  are  all  unsatisfactory,  since  the  water  will 
always  work  round  or  under  the  dam. 

Dew-ponds.  The  water  supply  of  upland  regions  lying  on 
the  Chalk  and  other  limestone  formations  presents  special 
difficulties.  From  remote  times  there  have  existed  on  the 


vii]  WATEK  SUPPLY  AND   DRAINAGE  177 

Chalk  downs  of  England  ponds  of  a  special  type,  known  as 
"dew-ponds,"  which  maintain  a  never-failing  supply  of  water 
under  the  most  unpromising  conditions,  and  there  has  been 
much  controversy  as  to  the  real  source  of  the  water  supply, 
which  is  obviously  not  due  to  springs,  since  such  are  non- 
existent on  the  tops  of  Chalk  hills.  This  problem  was  discussed 
in  1776  by  Gilbert  White  of  Selborne1,  who  drew  attention  to 
the  prevalence  of  heavy  mists  on  these  uplands,  even  in  the 
height  of  summer,  while  the  popular  name,  "dew-pond," 
testifies  to  the  general  belief  that  the  water  is  derived  directly 
from  the  atmosphere  by  a  process  similar  to  the  deposition  of 
dew  on  plants.  Of  late  years  it  has  been  shown  that  the  rainfall 
on  the  higher  parts  of  the  Downs  is  really  much  greater  than  on 
the  lower  ground  in  the  same  neighbourhood,  and  this  may  be 
of  more  importance  in  keeping  up  the  supply  than  is  commonly 
supposed. 

Dew-ponds  are  usually  constructed  as  follows:  an  excava- 
tion is  made  from  30  to  60  feet  in  diameter,  4  or  5  feet  deep  in 
the  middle,  with  sloping  sides.  This  is  first  lined  with  a  layer 
of  straw  or  brushwood  and  then  a  layer  of  puddled  clay  is 
placed  on  the  top  of  this.  It  is  generally  supposed  that  the 
straw  acts  as  an  insulator,  preventing  the  access  of  the  heat  of 
the  ground  to  the  layer  of  clay  and  to  the  water,  which  is 
cooled  by  radiation  and  thus  condenses  the  dew.  However  in 
Wiltshire  the  straw  is  often  put  on  the  top  of  the  clay,  instead 
of  below,  and  the  ponds  thus  constructed  seem  to  work  equally 
well.  It  is  evident  therefore  that  something  still  remains  to 
be  explained.  It  is  also  stated  that  newly-made  dew-ponds 
require  to  be  artificially  filled  at  first,  and  cannot  be  supplied 
by  condensation  of  moisture  on  the  dry  surface  of  the  clay  or 
straw  only,  or  even  from  the  rainfall.  Hence  we  must  suppose 
that  condensation  on  the  cold  water-surface  is  at  any  rate  an 
important  factor,  if  not  the  only  one  concerned. 

Rainfall  in  the  British  Isles.  Since  the  question  of  water 
supply  is  so  intimately  connected  with  the  amount  and  distri- 
bution of  rainfall,  it  may  be  well  to  consider  the  latter  a  little 
more  fully.  Within  the  limits  of  this  small  country  the  rainfall 

1  The  Natural  History  of  Selborne,  Letter  XXIX,  second  series,  1789 

B.  A.  G.  J2 


178  THE   GEOLOGY  OF  [CH. 

shows  remarkable  variations  in  amount,  and  this  variability  is 
directly  attributable  to  the  physical  structure  and  to  the 
geographical  conditions.  The  British  Isles  lie  in  the  temperate 
zone,  in  the  region  of  prevailing  south-westerly  winds.  This 
position  is  in  itself  favourable  to  abundant  rain,  since  the  air 
currents  which  have  picked  up  much  water  vapour  from  the 
tropical  Atlantic  are  here  entering  a  cooler  region,  and  thus 
becoming  able  to  carry  less  water  vapour,  that  is  to  say,  they  are 
approaching  the  point  of  saturation,  when  the  water  is  con- 
densed and  becomes  visible.  Hence  under  any  circumstances 
we  should  expect  a  copious  precipitation  in  this  region.  But 
from  special  causes,  this  precipitation  is  most  conspicuous  on 
the  western  side  of  the  country;  most  of  the  high  and  moun- 
tainous land  of  Britain  lies  to  the  west.  The  moist  winds  from 
the  Atlantic  blow  against  the  high  ground  and  the  air  is  driven 
upwards,  thus  being  cooled  and  becoming  saturated1.  Thus 
a  heavv  rainfall  prevails  on  the  mountains,  and  chiefly  on  the 
seaward  side.  When  the  air  descends  on  the  other  side  it- 
becomes  warmer  again,  and  also  actually  contains  less  moisture, 
since  it  has  parted  with  so  much  as  rain.  The  west  winds 
therefore  on  the  eastern  slopes  are  dry  winds,  tending  to  absorb 
moisture  instead  of  depositing  it.  Hence  they  may  also  be 
described  as  "drying"  winds.  It  follows  therefore,  from  geo- 
graphical considerations,  that  the  climate  of  eastern  England 
and  Scotland  must  be  much  drier  than  the  climate  of  the 
western  parts  of  the  same  countries. 

Along  the  eastern  coast  of  both  England  and  Scotland,  at 
certain  seasons  of  the  year,  easterly  and  south-easterly  winds 
bring  a  good  deal  of  rain,  but  when  the  east  winds  have  crossed 
the  main  watershed  and  descend  to  the  west  they  are  dry  and, 
being  warmed  by  the  descent,  they  bring  the  finest  weather. 
This  is  especially  the  case  in  spring  and  early  summer.  On  the 
east  coast  again  a  good  deal  of  moisture  is  deposited  by  the 
sea  fogs,  when,  although  it  is  not  actually  raining,  the  air  is 
saturated  with  moisture,  which  is  condensed  on  cool  surfaces, 
especially  on  plants.  This  phenomenon,  which  is  known  in 

1  The  amount  of  invisible  water  vapour  that  air  can  contain  is  directly 
proportional  to  the  temperature. 


vii]  WATER  SUPPLY  AND   DRAINAGE  179 

Scotland  as  a  "haar,"  contributes  materially  to  the  growth  of 
heavy  crops  in  the  east  of  Scotland  generally  and  especially 
in  the  highly  fertile  districts  of  the  Lothians,  the  Carse  of 
Gowrie  and  Aberdeen  shire.  The  cool  moist  atmosphere  thus 
produced  is  particularly  favourable  to  the  growth  of  turnips 
and  other  root  crops.  In  east  Yorkshire  also  the  root  crops  are 
as  a  rule  heavier  on  the  higher  ground  than  on  the  lowlands, 
owing  to  the  prevalence  of  sea  fogs  in  such  localities  during  the 
earlier  part  of  the  summer,  when  the  newly  germinated  seedling 
turnips  on  the  low  ground  are  liable  to  be  destroyed  by  drought 
and  the  turnip-beetle. 

When  we  come  to  deal  with  still  smaller  areas  the  same 
considerations  hold,  though  in  a  lesser  degree,  and  in  general 
terms  it  may  be  stated  that  hilly  country  has  a  higher  rainfall 
than  plains,  and  the  western  slopes  of  the  hills  have  more  rain 
than  the  eastern  slopes.  From  the  multiplication  of  observing 
stations  in  recent  years  it  is  becoming  clear  that  these  differences 
are  considerably  greater  than  was  formerly  believed  to  be  the 
case,  especially  with  regard  to  the  influence  of  elevation  on 
rainfall.  A  difference  of  a  very  few  hundred  feet  in  level  may 
make  a  vast  difference  in  the  records  of  a  rain-gauge.  It  is  a 
matter  of  common  observation  how  frequently  thunder-storms 
follow  even  slight  ridges,  and  the  same  is  true  in  a  less  degree 
of  the  general  annual  rainfall.  In  a  highly  civilized  country 
like  Britain  however  another  factor  has  to  be  taken  into  account, 
namely,  the  prevalence  in  certain  areas  of  a  smoky  atmosphere. 
It  is  a  well-known  fact  that  in  some  large  manufacturing  towns 
the  rainfall  is  disproportionately  great,  and  this  influence 
extends  for  some  distance  into  the  country  districts  round  them. 

So  far  as  the  actual  distribution  and  amount  of  rainfall  is 
concerned,  we  may  say  that  Ireland  is  very  wet,  except  in  the 
south-east  corner;  the  western  half  of  Scotland  is  also  very 
wet,  as  also  are  the  higher  parts  of  the  eastern  half.  If  we  draw 
a  line  from  the  mouth  of  the  Tyne  to  Bristol  and  then  to 
Exeter,  the  remaining  country  is  divided  into  two  parts,  wet 
on  the  west,  dry  on  the  east.  This  line  is  of  course  not  perfectly 
definite,  but  approximate  only,  and  there  is  a  long  dry  area 
projecting  into  the  Cheshire  plain,  mainly  owing  to  the  low 

12—2 


180  THE   GEOLOGY  OF  [CH. 

general  elevation  of  this  tract.  In  general  terms  it  may  be 
stated  that  to  the  west  of  this  line  the  average  annual  rainfall 
is  more  than  30  inches,  to  the  east  of  it  less  than  30  inches. 
But  in  many  parts  of  the  west  of  England  the  rainfall  is  far 
higher  than  this;  even  in  cultivated  and  low-lying  districts 
it  may  reach  60  inches,  while  among  the  mountains  the  figure 
may  reach  twice  this  amount.  The  highest  figure  for  many 
years  anywhere  recorded  by  trustworthy  instruments  is  at 
Seathwaite  in  Cumberland,  where  the  annual  average  is  about 
130  inches  and  in  wet  years  over  200  inches  have  been  measured. 
On  the  other  hand,  in  the  low-lying  districts  bordering  the  east 
coast,  especially  in  Lincolnshire  and  Essex,  the  climate  is  very 
dry.  At  some  places  on  the  Thames  estuary  the  annual  average 
is  as  low  as  18  or  19  inches,  while  round  the  Wash  it  is  not  much 
more.  Over  eastern  England  generally,  south  of  the  Tyne,  the 
rainfall  may  be  taken  at  about  25  inches.  This  would  appear 
at  first  sight  to  be  scarcely  sufficient  for  successful  agriculture, 
and  in  fact  this  district  in  certain  years,  such  as  1911,  does  suffer 
from  prolonged  droughts.  But  in  most  years  it  is  found  that 
a  considerable  proportion  of  the  rain  falls  in  the  summer  months ; 
in  fact  over  a  large  area  in  Suffolk,  Essex  and  Cambridge- 
shire, July  is  the  wettest  month  of  the  whole  year;  this  is 
probably  to  be  attributed  to  the  prevalence  of  heavy  thunder- 
storms at  that  time.  These  are  often  accompanied  by  hail  of 
such  large  size  as  to  be  seriously  destructive  to  crops.  Hail- 
stones as  large  as  a  pigeon's  egg,  or  even  a  small  hen's  egg, 
are  not  uncommon. 

It  is  evident  that  variations  of  such  extent,  amounting  to 
three  or  four  hundred  per  cent,  of  the  rainfall,  must  have  an 
important  influence  on  agriculture.  A  heavy  rainfall  favours 
the  growth  of  most  crops,  but  on  the  other  hand  it  renders 
harvesting  difficult  and  precarious.  The  wet  districts  are  on 
the  whole  later  than  the  dry  districts,  though  the  crops  are  often 
heavier.  This  applies  more  particularly  to  grass,  green  crops 
and  roots.  Wheat  on  the  other  hand  prefers  a  hot  dry  climate, 
if  the  soil  is  not  too  light,  and  barley  is  certainly  of  better  quality 
if  the  air  is  not  too  moist.  But  the  most  important  influence 
of  rainfall,  although  this  is  a  fact  not  yet  generally  recognized, 


vn]  WATER  SUPPLY  AND  DRAINAGE  181 

is  on  the  soil  itself,  and  especially  on  the  stores  of  available 
plant  food.  The  researches  of  foreign  investigators,  especially 
in  Russia,  have  shown  how  far-reaching  is  the  influence  of 
"washing  out",  or  "leaching  out"  of  the  soluble  constituents  of 
the  soil  in  moist  climates.  This  constitutes  for  example  the 
special  characteristic  of  the  Podzol  type  of  soil,  which  is  so  widely 
spread  in  Russia  (see  Chapter  vi),  and  according  to  Ramann 
the  soils  in  the  western  part  of  the  British  Isles  approximate 
to  this  type,  while  in  the  east  they  belong  to  the  "brown-soil" 
type1.  Although  the  differences  here  alluded  to  are  not  very 
well  marked  in  this  country  they  probably  exist.  The  chief 
lesson  to  be  learnt  from  these  considerations  is  the  desirability 
in  a  moist  climate  of  keeping  the  soil  well  occupied  by  crops, 
avoiding  fallows  as  much  as  possible,  since  when  the  soil  is 
bare  there  must  necessarily  be  a  great  loss  of  soluble  material, 
especially  nitrogen  and  lime.  On  the  other  hand  in  a  dry  climate 
a  bare  summer  fallow  may  actually  lead  to  an  increase  of  plant 
food  by  the  rise  of  solutions  from  below.  In  extreme  cases 
such  rising  ground-water  may  bring  up  deleterious  salts,  as  in 
the  alkali  soils  of  western  America  and  other  arid  regions,  but 
in  Britain  this  does  not  happen. 

1  Ramann,  Bodenkunde,  3rd  edition,  Berlin,  1911,  p.  582. 


CHAPTER  VIII 

GEOLOGICAL  MAPS  AND  SECTIONS 

In  agricultural  geology,  no  less  than  in  other  branches  of  the 
subject,  the  study  of  maps  is  of  first  rate  importance.  From 
an  ordinary  topographical  map,  without  geological  lines,  it 
is  possible  to  obtain  much  information  of  use  to  the  farmer 
with  regard  to  position,  elevation  and  aspect  of  the  land, 
drainage  and  water  supply,  accessibility  by  road,  rail  or  river, 
distance  from  market  towns  and  centres  of  population  and  many 
other  points  that  may  be  of  interest  in  view  of  a  possible  tenancy 
or  ownership  in  a  new  district.  Before  inspecting  a  farm  a 
good  map  should  always  be  consulted,  as  it  may  be  at  once 
apparent  that  some  insuperable  drawback  exists,  or  that  the 
situation  offers  special  advantages.  All  this  is  not  geological 
and  does  not  need  much  expert  knowledge.  But  the  study  of 
geological  maps  with  a  view  to  forming  an  opinion  as  to  the 
possible  quality  of  the  soil  is  a  different  matter,  and  requires 
some  preliminary  acquaintance  with  the  subject. 

Geological  maps  may  be  actually  misleading  as  to  the  char- 
acter of  the  surface  soil,  unless  certain  important  principles  are 
carefully  borne  in  mind.  The  first  point  is  in  regard  to  variations 
in  the  lithological  characters  of  rocks  of  the  same  age  when 
followed  from  place  to  place. 

In  nearly  all  maps  the  rocks  of  a  given  age  are  indicated 
everywhere  by  the  same  colour  or  sign,  regardless  of  change  in 
their  characters.  Again,  some  maps  show  the  superficial 
deposits  as  defined  in  Chapter  v,  while  others  ignore  them 
altogether;  another  class  of  maps  shows  some  of  them  and  not 
others;  this  is  perhaps  the  most  misleading  of  all,  though 
unfortunately  very  common.  It  is  hardly  necessary  to  insist 


CH.  vm]    GEOLOGICAL  MAPS  AND   SECTIONS  183 

on  the  obvious  fact  that  the  farmer  is  most  concerned  with  the 
uppermost  layer  of  the  earth's  crust  and  that  what  lies  beyond 
the  reach  of  the  roots  of  crops  is  of  secondary  and  indirect 
interest  only.  Then  again  most  geological  maps  are  on  too 
small  a  scale  to  indicate  the  variations  that  are  likely  to  occur 
within  the  limits  of  a  single  farm,  at  any  rate  in  this  country, 
where  really  large  farms  are  exceptional.  In  spite  of  all  these 
drawbacks,  however,  the  study  of  geological  maps  when  properly 
understood,  is  of  great  use  to  the  landowner  and  farmer  and 
may  afford  valuable  information  on  certain  matters  of  great 
practical  importance,  such  as  drainage,  water  supply,  the 
occurrence  of  building-stone,  road-metal,  limestone,  chalk, 
marl,  gravel  and  so  forth,  as  well  as  with  regard  to  the  probable 
nature  of  the  soil. 

Topographical  maps.  Of  these  there  are  almost  innumerable 
varieties,  on  all  scales.  In  some  of  them  no  attempt  is  made 
to  indicate  the  relief  of  the  surface,  little  being  shown  besides 
coast  lines,  rivers,  railways,  roads,  towns,  villages  and  political 
and  municipal  boundaries  of  various  kinds.  To  this  category 
belong  most  of  the  maps  in  atlases.  In  some  of  these  the  relief 
is  partly  indicated  by  the  method  of  hill-shading,  a  term  which 
explains  itself.  Most  of  these  are  on  a  small  scale,  indicating 
geographical  position  and  little  more.  Of  much  greater  use 
are  the  maps  which  show  the  relief  of  the  surface  by  the  use  of 
contour-lines,  and  a  special  variety  of  contour  maps,  known  as 
the  layer  system.  A  contour-line  is  an  imaginary  line  passing 
through  all  points  on  the  surface  that  are  at  equal  heights 
above  sea-level,  usually,  in  this  country,  some  multiple  of  one 
hundred  feet.  In  the  layer  maps  the  spaces  enclosed  between 
two  successive  contour-lines  are  coloured  with  a  tint  corre- 
sponding to  a  definite  range  of  height.  For  example  in  the 
excellent  maps  published  by  Messrs  Bartholomew  &  Co.  of 
Edinburgh,  on  the  scale  of  2  miles  to  1  inch  all  the  land  up  to 
400  feet  is  coloured  in  shades  of  green,  the  deepest  tint  at  the 
lowest  levels.  Above  400  feet  brown  tints  are  employed, 
increasing  in  depth  upwards.  By  this  means  the  relief  of  the 
land  and  the  distribution  of  hill  and  valley  are  well  brought  out, 
and  even  on  this  small  scale  it  is  easy  to  make  out  the  height 


184  GEOLOGICAL  MAPS   AND   SECTIONS  [CH. 

and  aspect  of  any  elevated  or  inclined  land,  and  even  the  steep- 
ness or  otherwise  of  the  slopes,  by  taking  into  account  the 
narrowness  or  breadth  of  the  colour  bands.  It  must  however 
be  remembered  that  the  intervals  between  successive  contour- 
lines  are  not  constant,  but  usually  increase  at  higher  elevations. 

For  most  purposes  the  best  maps  are  those  published 
officially  by  the  Ordnance  Survey  of  the  United  Kingdom. 
Of  these  the  most  generally  useful  are  those  on  the  scales  of 
1  inch  to  the  mile,  6  inches  to  the  mile  and  25-344  inches  to 
the  mile.  The  1-inch  map,  which  can  be  obtained  with  contour- 
lines  or  with  hill-shading,  is  useful  for  a  general  view  of  a  large 
district,  showing  roads,  railways,  towns  and  villages.  The 
6-inch  map  may  be  used  for  plans  of  large  estates,  but  is 
too  small  for  details.  This  map  also  shows  contour-lines,  at 
intervals  of  100  feet  up  to  1000  feet  and  above  this  at  wider 
intervals.  The  50  foot  contour  is  also  shown.  The  6-inch 
maps  of  Yorkshire  and  Lancashire  are  contoured  at  every 
25  feet,  instead  of  100  feet,  and  these  show  small  variations 
of  the  surface  very  accurately.  For  plans  of  single  farms  the 
third  variety,  commonly  called  the  25-inch  map,  is  the  most 
useful.  On  this  the  acreage  of  every  field  is  indicated,  but  there 
are  no  contour-lines.  On  these  maps  individual  fields  are  large 
enough  to  afford  space  for  notes  as  to  the  succession  of  crops 
or  any  other  information  likely  to  be  of  value,  and  every  field 
and  enclosure  is  numbered.  This  map  is  also  excellent  for 
plotting  the  results  of  detailed  soil-surveys.  There  are  also 
maps  on  a  larger  scale  than  this,  but  these  are  only  available 
for  towns. 

Geological  maps.  There  are  in  existence  many  small-scale 
geological  maps  of  the  whole  of  the  British  Isles,  of  England 
and  Wales,  of  Scotland,  of  Ireland,  and  of  most  other  civilized 
countries.  The  majority  of  these  are,  however,  apart  from  their 
small  size,  too  generalized  to  be  of  much  service  to  agricul- 
turists, especially  as  very  few  of  them  take  any  cognizance  of 
the  superficial  deposits.  They  serve  only  to  give  the  most 
general  idea  of  the  geological  formations  that  underlie  any 
district,  and  afford  little  information  as  to  soils.  Small  maps 
of  similar  character  for  special  areas  are  also  to  be  found  in 


vm]          GEOLOGICAL   MAPS  AND   SECTIONS  185 

many  county  histories,  county  geographies  and  similar  publi- 
cations1. 

The  most  generally  useful  geological  maps  of  the  United 
Kingdom  are  ,  those  published  officially  by  the  Geological 
Surveys  of  the  United  Kingdom  and  of  Ireland.  Maps  of  the 
whole  of  the  British  Isles  (except  some  small  portions  of  the 
Highlands  of  Scotland,  still  incomplete)  are  published  on  the 
scale  of  1  inch  to  the  mile,  and  of  these  there  are  two  series: 
the  "solid"  maps  show  only  the  rock-formations,  ignoring  the 
superficial  deposits,  except  certain  large  areas  of  alluvium  and 
peat;  the  other  series,  the  "drift"  maps,  show  the  superficial 
deposits,  paying  special  attention  to  the  accumulations  of 
glacial  origin,  and  to  the  gravels  of  post-glacial  date.  In  spite 
of  some  inconsistencies  of  treatment,  especially  among  the 
earlier  published  maps,  these  are  most  useful  for  purposes  of 
soil  study,  although  the  scale  is  small.  Maps  of  a  considerable 
part  of  southern  England  and  some  other  special  regions  are 
now  issued  in  a  new  and  revised  colour-printed  edition.  These 
are  cheap  and  excellent. 

Geological  maps  on  the  scale  of  6  inches  to  the  mile  have 
also  been  published  for  certain  districts,  chiefly  those  where 
mining  is  important.  Manuscript  6-inch  maps  of  the  whole 
country  have  been  prepared  and  are  preserved  in  the  Museum 
of  Practical  Geology,  Jermyn  Street,  London,  where  they  can 
be  consulted  on  application. 

A  so-called  "Index  Map"  of  England  and  Wales  is  also 
published  by  the  Geological  Survey,  on  the  scale  of  4  miles  to 
1  inch.  This  is  now  undergoing  revision,  and  some  sheets  are 
now  published  in  a  "drift"  edition.  These  show  a  wonderful 
amount  of  detail  considering  the  small  scale2. 

For  different  kinds  of  maps  various  methods  of  projection 
are  employed,  according  to  circumstances,  varying  with  the 
size  of  the  area  depicted.  All  large  scale  maps  are  projections 

1  Special  mention  may  be  made  of  the  excellent  small  geological  maps  given 
in  the  Cambridge  County  Geographies,  published  by  the  Cambridge  University 
Press. 

2  A  complete  catalogue  of  the  publications  of  the  Ordnance  Survey  and  of 
the  Geological  Survey  can   be  obtained  from  Messrs  Edward  Stanford,  Long 
Acre,  London,  W.C. 


186  GEOLOGICAL  MAPS   AND   SECTIONS  [CH. 

on  a  horizontal  plane  of  a  portion  of  the  earth's  surface  as  it 
would  be  seen  from  an  infinite  distance.  Hence  if  it  is  required 
to  indicate  the  relief  of  the  surface  the  method  of  contour-lines 
must  be  employed.  Let  us  consider  for  the  sake  of  simplicity 
a  region  of  moderate  relief  and  elevation,  for  example,  some 
part  of  England  less  than  1000  feet  above  sea-level.  The  pub- 
lished 6-inch  Ordnance  maps  of  such  a  region  show  contour-lines 
at  intervals  of  100  feet.  Let  it  be  assumed  also  that  the  region 
is  free  from  superficial  deposits,  such  as  alluvium,  gravel  or 
peat,  and  consists  wholly  of  stratified  sedimentary  rocks. 

The  first  and  simplest  case  is  where  the  strata  are  horizontal ; 
here  it  is  evident  that  the  outcrops  will  be  parallel  to  the 
contour-lines,  though  the  dividing  lines  between  the  different 
strata  do  not  necessarily  coincide  with  contour-lines;  in  fact 
they  commonly  do  not.  When  the  outcrops  in  such  a  map  are 
coloured  with  appropriate  tints  for  each  stratigraphical  division 
the  general  effect  resembles  the  "layer  maps"  already  mentioned. 
A  close  approximation  to  this  type  is  to  be  seen  in  some  of  the 
published  maps  representing  parts  of  the  Carboniferous  area 
of  west  Yorkshire,  or  parts  of  the  Cretaceous  outcrops  in  the 
south  of  Cambridgeshire.  The  width  of  the  outcrop  of  a  par- 
ticular stratum  as  projected  on  the  map  is  due  to  a  combination 
of  two  factors,  namely, 

(1)  the  actual  vertical  thickness  of  the  bed, 

(2)  the  angle  of  slope  of  the  ground. 

If  the  surface  of  the  ground  is  uniformly  horizontal,  only 
the  uppermost  stratum  can  appear,  and  the  width  of  its  outcrop 
is  indefinite,  being  determined  only  by  the  area  of  the  plain. 
On  the  other  hand,  in  a  vertical  cliff,  all  strata  below  the  highest 
will  be  invisible  on  the  map,  or  as  it  may  be  otherwise  expressed 
the  width  of  their  outcrop  is  zero.  Between  these  two  limiting 
values  of  zero  and  infinity  the  outcrop  may  have  any  width 
whatever.  In  practice,  since  most  ground  is  undulating  and 
most  strata  not  very  thick,  the  width  of  the  outcrop  is  usually 
confined  within  fairly  narrow  limits.  Since  the  three  quantities, 
width  of  outcrop,  slope  of  ground  and  thickness  of  stratum  are 
connected  by  a  definite  mathematical  relationship,  knowing  any 
two  of  them,  we  can  calculate  the  third. 


vin]  GEOLOGICAL   MAPS  AND   SECTIONS  187 

The  problem  may  present  itself  in  the  following  practical 
form:  In  the  figure  let  ABC  represent  a  section  through  a  hill 
composed  of  horizontal  strata  of  varying  character ;  the  surface 
from  A  to  B  is  covered  by  soil  and  rainwash,  so  that  no  solid 
rock  is  visible ;  at  C  a  well  has  been  sunk  and  the  thickness  of 
the  beds  noted ;  it  is  required  to  find  the  width  of  the  outcrop 
of  the  shaded  bed  on  the  slope  AB.  From  the  construction 
it  is  clear  that  the  width  of  outcrop  required  when  plotted  on 
the  map  is  given  by  D'E' ',  whereas  when  measured  on  the  ground 
from  D  to  E  it  will  be  slightly  greater,  in  a  ratio  determined  by 
the  angle  of  inclination  of  the  hill;  the  steeper  the  hill,  the 
greater  this  difference  will  be1.  This  method  may  give  valuable 
information  as  to  the  best  place  to  open  a  quarry,  and  in  many 
other  cases.  It  is  evidently  equally  applicable  with  modifica- 
tions to  inclined  strata. 


A'  D'  E'  C 

Fig.  41.  ABC  is  the  outline  of  the  hill,  seen  in  section,  A'C'  a  horizontal  line 
drawn  at  any  convenient  distance  below  the  surface,  C  is  a  well.  D  and  E  the 
limits  of  the  outcrop  of  the  stratum,  D'E'  is  the  width  of  the  outcrop  as 
plotted  on  the  map. 

A  similar  plan  can  be  employed  in  undulating  country, 
composed  of  a  succession  of  hills  and  valleys;  the  outcrop  of 
a  bed  can  be  continued  from  one  hillside  to  another,  across  an 
intervening  valley,  and  when  the  strata  are  inclined  such  a 
section,  if  correctly  drawn,  will  show  whether  a  particular 
stratum  should  occur  on  the  slopes  of  a  neighbouring  hill,  or 
whether  the  dip  has  carried  it  either  below  ground-level  or  up 
into  the  air  above  the  summit  of  the  hill.  For  this  purpose, 
as  indeed  in  all  other  instances  here  mentioned,  it  is  of  course 
obvious  that  the  section  must  be  drawn  to  true  scale,  and  as 
accurately  as  possible.  The  dips  also  must  be  measured  with 

1  If  the  inclination  of  the  slope  DE  to  the  horizontal  plane  be  6°,  then  since 
D'E'  =  DF,  therefore  D'E'  =  DE  cos  6;  similarly  EF  =  DE  sin  6. 


188  GEOLOGICAL  MAPS  AND   SECTIONS  [CH. 

care  and  plotted  with  a  protractor.  The  instrument  commonly 
used  for  measuring  the  dip  of  strata  is  called  a  clinometer: 
there  are  many  forms  on  the  market,  the  simpler  ones  being  the 
best.  The  essential  feature  is  a  straight  edge  fixed  to  a 
graduated  circle,  marked  in  degrees,  with  a  movable  plumb-line 
or  pendulum.  The  straight  edge  is  laid  on  the  inclined  rock- 
face,  down  its  steepest  slope,  the  angle  made  with  the  vertical 
by  the  plumb-line  being  read  off  directly  from  the  circle.  The 
clinometer  is  often  combined  with  a  compass  for  determining 
(by  separate  observations)  the  direction  as  well  as  the  amount 
of  the  dip. 

Hitherto  it  has  been  assumed  that  the  thickness  of  the  various 
strata,  as  well  as  their  dips,  if  inclined,  remains  uniform,  but 
of  course  this  is  not  always  the  case.  Many  rock-beds  are 
lenticular  in  form  and  of  limited  extent,  thinning  out  and 
disappearing  within  a  greater  or  less  distance.  In  such  cases 
observations  of  the  dip  of  both  upper  and  lower  surfaces  will 
often  give  useful  information.  If  the  surfaces  are  curved 
however  the  matter  becomes  more  difficult.  Again  in  many 
regions  great  complication  is  introduced  by  folding,  faulting, 
and  other  dislocations. 

In  horizontal  strata  the  only  disturbances  likely  to  occur 
are  due  to  faulting.  This  is  a  complex  subject,  but  one  or  two 
simple  cases  may  be  considered  briefly.  The  general  principles 
of  faulting  have  been  discussed  in  Chapter  i,  where  the  terms 
in  use  are  also  defined.  The  easiest  case  is  where  the  fault  is 
vertical;  on  sloping  ground  the  effect  of  this  will  be  to  bring 
the  outcrop  to  a  sudden  end;  on  level  ground  also  this  is  the 
only  thing  that  can  happen.  On  a  slope  however  the  direction 
and  amount  of  the  throw  of  the  fault  may  be  such  as  to  cause 
a  repetition  of  the  outcrop.  The  two  cases  are  illustrated  in 
Fig.  42.  From  these  diagrams  it  is  clear  that  when  the  upthrow 
of  the  fault  is  on  the  left  or  down-hill  side,  there  can  be  no 
further  outcrop  unless  the  ground  rises  again  in  this  direction. 
When  the  downthrow  of  the  fault  is  on  this  side  the  outcrop 
of  the  bed  may  be  repeated,  or  it  may  drop  so  far  below  ground- 
level  as  to  disappear  altogether.  However  if  it  does  this  the 
bed  might  be  reached  by  boring.  In  nature  it  is  by  far  the  more 


VIII] 


GEOLOGICAL  MAPS  AND  SECTIONS 


189 


common  to  find  the  downthrow  on  the  down-hill  side,  the 
converse  case  being  rather  rare.  It  is  often  possible,  by  studying 
the  strata  above  and  below  the  one  employed  as  an  indicator, 
to  ascertain  what  has  happened,  since  some  of  them  are  bound 
to  come  to  the  surface  somewhere  on  both  sides  of  the  fault, 
although  as  a  little  consideration  will  show,  a  particular  stratum 
may  be  "faulted  out"  altogether,  that  is  to  say,  it  may  never 


'  .   i. 


J — I i      i       L 


Vertical  fault  with  upthrow  to  the  left. 


Vertical  fault  with  downthrow  to  the  left 
Fig  42. 

intersect  the  surface  at  all.  When  dealing  with  problems 
connected  with  faulting  the  methods  briefly  outlined  above  can 
be  applied  successfully  to  the  solving  of  the  simpler  problems. 
The  effects  of  faulting  of  various  kinds  in  repeating  and 
suppressing  the  outcrops  of  inclined  strata  are  dealt  with  in 
Chapter  i  (see  p.  23  and  Figs.  12  and  15). 

When  dealing  with  inclined  strata,  in  addition  to  the  slope 
of  the  ground  and  the  thickness  of  the  beds,  we  have  to  take 
into  account  also  the  direction  and  amount  of  the  dip.  In 
inclined  strata  the  thickness  is  not  measured  vertically,  but  it 
is  the  least  distance  between  the  upper  and  lower  surfaces  of 
the  bed,  perpendicular  to  the  bedding  planes  whatever  the 
inclination  (dip)  may  be.  If  the  beds  are  vertical  therefore  the 
thickness  is  measured  horizontally.  As  a  matter  of  convenience 
vertical  strata  may  be  considered  first.  Here  outcrop  and 


190  GEOLOGICAL  MAPS   AND   SECTIONS  [CH. 

strike1  coincide  and  the  width  of  the  outcrop,  as  plotted  on  the 
map,  can  never  be  less  or  greater  than  the  true  thickness  of  the 
bed.  The  outcrops  of  vertical  strata  will  of  course  undulate 
in  the  vertical  plane  when  followed  over  hills  and  valleys,  but 
when  correctly  proj  ected  on  a  map  they  must  appear  as  straight 
parallel  lines,  so  long  as  the  thicknesses  remain  uniform.  Exactly 
similar  statements  apply  to  fault-planes;  a  straight  vertical 
fault  must  appear  on  a  map  as  a  straight  line ;  many  faults  are 
however  curved,  and  such  appear  on  maps  as  sinuous  lines, 
hence  it  is  not  always  possible  to  tell  by  inspection  of  a  map 
whether  a  fault  is  vertical  or  not;  a  perfectly  straight  fault 
indicated  on  sloping  surfaces  must  be  vertical,  but  a  curved 
outcrop  of  a  fault  may  indicate  either  actual  curvature  or  a 
departure  from  verticality. 

When  the  strata  are  inclined  at  any  angle  between  0°  and 
90°  the  width  of  the  outcrop  on  the  map  varies  indefinitely,  the 
three  independent  variables  being  the  thickness  of  the  bed  and 
the  angles  of  inclination  of  the  surface  and  of  the  bed. 

The  simplest  case  is  where  the  ground  is  horizontal,  since 
one  variable  is  here  eliminated,  and  the  width  depends  only  on 
the  thickness  and  dip,  being  wider  with  lower  angles  of  dip. 
Fig.  43  shows  the  relations  between  thickness,  dip  and  width 
of  outcrop  in  an  inclined  stratum. 


Fig.  43.     BD  =  upper  surface  of  bed,  AC  =  lower  surface,  5(7  =  thickness 
of  bed,  AB  =  width  of  outcrop,  BAG  =  angle  of  dip. 

Again  when  the  dip  and  thickness  are  uniform  it  is  easy  to 
show  that  the  width  of  the  outcrop  as  seen  on  the  map  depends 
on  the  slope  of  the  ground,  being  widest  on  a  level  plain  and 
vanishing  on  a  vertical  face.  If  the  bed  dips  the  same  way 
as  the  surface  of  the  ground,  the  width  of  the  outcrop  is 
indefinite. 

1  For  definitions  of  dip  and  strike  see  p.  1.7. 


vm]  GEOLOGICAL  MAPS  AND  SECTIONS  191 

When  dealing  with  inclined  strata  outcropping  on  inclined 
ground  there  are  thus  three  variable  factors  to  be  taken  into 
account,  namely,  dip,  thickness,  and  slope  of  the  ground. 
Since  however  these  are  obviously  connected  by  simple  mathe- 
matical relations,  if  any  two  are  known  it  is  easy  to  find  the 
third  by  calculation  or  by  graphic  methods ;  the  latter  are 
generally  employed.  The  question  evidently  becomes  more 
complex  when  the  slope  of  the  ground  is  variable,  and  especially 
when  the  ground  slopes  the  same  way  as  the  bed.  Each  such 
instance  must  generally  be  dealt  with  independently  and  no 
general  rule  can  be  given.  The  only  method  that  gives  a 
reliable  result  is  to  make  a  profile  section  of  the  ground  to  true 
scale,  from  the  contour-lines,  and  then  to  insert  the  geological 
structure,  with  the  correct  dip  and  thickness;  the  section  will 
then  show  where  any  particular  stratum  may  be  looked  for,  if 
its  outcrop  is  covered  by  drift,  rainwash  or  any  other  superficial 
accumulation.  This  method  will  often  also  give  useful  indica- 
tions of  the  probable  thickness  of  such  superficial  deposits  at 
any  given  point.  For  instance,  if  there  is  reason  to  believe 
that  an  old  valley  has  been  partly  filled  up  by  later  deposits, 
thus  giving  steep  sides  and  a  flat  floor,  it  may  be  possible  by 
continuing  downwards  the  slope  of  the  steep  upper  part,  to 
obtain  a  rough  estimate  of  the  probable  depth  from  the  surface 
of  the  old  sides  of  the  valley.  Such  a  section,  to  be  of  any 
use,  must  evidently  be  drawn  to  true  scale. 

The  next  point  to  be  considered  is  the  form  of  the  outcrops 
of  inclined  strata  on  undulating  ground.  It  has  already  been 
shown  that  vertical  strata  have  straight  outcrops,  while  the 
outcrops  of  horizontal  strata  are  parallel  to  the  contour-lines; 
hence  for  any  intermediate  inclination  the  outcrops  must  be 
sinuous  lines,  cutting  the  contour-lines.  The  direction  of  the 
sinuosities  depends  on  the  form  of  the  surface.  The  relations 
are  most  easily  seen  by  means  of  a  solid  model  representing  a 
river  valley  carved  out  of  inclined  strata.  When  the  strata 
dip  towards  the  head  of  the  valley  their  outcrops  will  have 
a  V-shaped  form,  the  apex  of  the  V  pointing  up  the  valley; 
when  the  dip  is  down  stream,  at  an  angle  greater  than  the 
slope  of  the  stream,  the  apex  of  the  V  will  point  downwards. 


192  GEOLOGICAL  MAPS   AND   SECTIONS     [CH.  vm 

When  the  dip  of  the  beds  is  down  the  valley  at  an  angle  less 
than  the  slope  of  the  stream,  the  apex  of  the  V  will  however 
point  up  stream.  This  case  is  less  common  than  the  other  two. 
By  taking  account  of  these  facts  it  is  usually  easy  to  determine 
by  inspection  the  direction  of  dip  of  inclined  strata. 

When  a  geological  map  showing  contours  is  drawn  accurately 
to  scale  it  is  possible  to  determine  by  purely  graphical  methods 
the  dip  and  thicknesses  of  the  strata  shown,  to  find  the  depth 
from  the  surface  at  which  a  given  bed  will  be  found  at  a 
particular  place,  to  measure  the  throw  of  faults  and  to  obtain 
much  information  of  value  to  practical  men.  It  is  impossible 
however,  owing  to  limitations  of  space,  to  enter  here  into  a 
description  of  the  methods  to  be  employed. 


CHAPTER   IX 

STRATIGRAPHICAL  GEOLOGY.     INTRODUCTION 

For  the  agriculturist  the  most  important  application  of 
geology  is  the  study  of  the  geographical  distribution  of  the 
different  types  of  rock  and  their  relations  to  crops  and  stock. 
It  is  well  known  to  every  one  that  in  different  parts  of  any 
given  country  different  kinds  of  farming  prevail ;  in  one  area 
nearly  all  the  land  is  under  the  plough,  in  another  it  is  mainly 
permanent  pasture,  while  in  other  areas  again,  especially 
high-lying  and  mountainous  regions,  the  land  is  mostly  unen- 
closed, being  devoted  to  grazing,  or  even  entirely  unoccupied 
by  domesticated  animals,  though  in  Britain  this  condition  is 
rare.  In  determining  the  agricultural  potentialities  of  a  district 
geological  structure  and  the  characters  of  the  rocks  are  of 
predominating  influence,  though  other  factors  play  an  important 
part,  such  as  climate,  economic  conditions,  and  even  the  mental 
and  moral  characteristics  of  the  people.  The  effect  of  climate 
is  undoubtedly  very  great  and  in  the  British  Isles  it  is  specially 
well  marked,  owing  to  the  great  differences  in  the  temperature 
and  rainfall  on  the  eastern  and  western  sides  of  the  country. 
The  wet  climate  of  the  western  side  is  largely  attributable  to 
the  superior  elevation  of  the  land,  and  this  is  due  in  the  first 
place  to  the  geological  structure,  most  of  the  older  and  harder 
rocks  occurring  on  that  side  and  tending  to  form  high  ground. 
Thus  it  is  clear  that  some  of  these  factors  are  mutually  inter- 
dependent, and  it  is  often  very  difficult  or  even  impossible  to 
disentangle  their  effects.  The  influence  of  latitude  has  also  to 
be  taken  into  account,  although  it  appears  that  in  the  temperate 
zone  at  any  rate,  this  is  comparatively  small.  For  instance  in 
central  Norway,  only  a  very  few  degrees  from  the  arctic  circle, 

R.  A.  G.  13 


194  STRATIGRAPHICAL   GEOLOGY  [OH. 

luxuriant  forests  grow  and  farm  crops  ripen  up  to  a  much 
greater  height  above  sea-level  than  in  Scotland  or  Ireland,  8 
or  10  degrees  further  south.  In  the  warmer  regions  of  the  world, 
in  the  tropical  and  subtropical  zones,  the  rainfall  is  the  chief 
factor,  and  in  no  subject  more  than  in  agricultural  geology  do 
we  realize  the  importance  of  the  existence  of  climatic  belts,  as 
explained  in  Chapter  n.  However  rich  a  soil  may  be,  if  dry  it 
is  barren.  Hence  the  enormous  importance  of  irrigation  in 
Australia,  South  Africa,  and  western  America,  where  there  are 
often  great  stores  of  underground  water  waiting  to  be  tapped, 
or  streams  running  from  mountain  ranges  through  sandy 
wastes,  and  only  needing  to  be  properly  distributed  to  yield 
surprising  results  in  the  way  of  heavy  crops  from  soils  of  most 
unpromising  appearance. 

To  sum  up,  the  suitability  of  a  region  for  farming  depends 
not  only  on  the  character  and  distribution  of  the  rocks,  but 
also  on  latitude,  climate  and  other  factors  of  minor  importance. 

The  object  of  the  present  and  succeeding  chapters  is  to 
study  the  characters  and  geographical  distribution  of  the  rocks 
and  other  deposits  that  compose  the  outer  crust  of  the  earth, 
forming  the  surface  of  the  ground  and  giving  rise  by  their 
decomposition  and  decay  to  the  cultivated  soil  on  which  the 
farmer  grows  his  crops  and  pastures  his  stock.  Since  this 
book  is  primarily  intended  for  English  readers  most  attention 
will  be  paid  to  the  rock-formations  as  developed  in  the  British 
Isles,  but  incidental  reference  will  be  made  to  foreign  and 
colonial  examples  in  illustration  of  special  points. 

The  agricultural  importance  of  any  particular  formation 
depends  on  a  large  number  of  independent  and  variable  factors. 
Thus  for  example  a  thick  succession  of  rocks  in  a  vertical  or 
highly  inclined  position  may  form  a  much  less  area  of  the  surface 
of  a  country  than  a  thin  series  spread  out  in  horizontal  or  very 
slightly  inclined  layers;  again  a  thin  series,  though  highly 
inclined,  may  be  repeated  many  times  by  folding.  But  of  still 
more  importance  is  the  fact  that  the  solid  rocks  of  the  crust  are 
often  concealed  from  view  by  a  layer  of  transported  material, 
drift  or  alluvium ;  in  such  a  case  it  is  the  nature  of  the  super- 
ficial layer  that  is  of  interest  to  the  farmer,  since  this  determines 


ix]  STRATIGRAPHICAL  GEOLOGY  195 

the  nature  of  the  soil.  It  is  only  the  uppermost  layer  of  all 
that  forms  the  cultivated  soil;  however  rich  in  plant  food  the 
underlying  materials  may  be  this  is  useless  if  covered  up  by 
a  very  few  feet  of  unworkable  or  unproductive  material. 
A  striking  example  of  this  is  afforded  by  the  surface  quartzites 
and  surface  limestones  of  South  Africa  and  other  dry  regions, 
described  on  p.  117. 

A  consideration  of  such  facts  shows  at  once  the  critical 
importance  of  the  careful  investigation  and  mapping  of  the 
superficial  deposits,  and  the  comparative  neglect  of  these  has 
rendered  many  geological  maps  worse  than  useless  to  the  farmer. 
A  map  showing  only  what  the  geologist  calls  the  solid  formations 
is  merely  misleading,  since  the  strata  shown  on  the  map  may 
actually  be  buried  under  tens  or  even  hundreds  of  feet  of 
superficial  deposits  of  entirely  different  character.  It  is  only 
within  recent  years  that  true  soil-surveys,  confined  to  the 
workable  soil  itself,  have  been  carried  out  in  certain  limited 
areas.  The  most  useful  maps  for  agricultural  purposes  are  the 
u  Drift"  series  published  by  the  Geological  Survey,  but  even 
in  these  the  scale  is  only  1  inch  to  the  mile,  and  very  different 
types  of  superficial  deposit  are  often  indicated  by  the  same 
colour.  A  simple  designation,  such  as  boulder-clay,  includes  a 
great  variety  of  clays  and  more  or  less  sandy  beds  formed  by 
glaciers  and  ice-sheets,  while  gravels  and  alluvium  often  differ 
a  good  deal  in  composition  and  agricultural  character.  It  is 
only  in  the  south  and  south-west  of  England  that  glacial 
deposits  are  wholly  absent  over  large  areas  and  here  the  "solid" 
maps  indicate  with  precision  the  character  of  the  soils. 

After  due  allowance  has  been  made  for  all  the  difficulties 
detailed  above,  the  fact  remains  that  a  geological  map  on  a 
sufficiently  large  scale  does  afford  a  general  indication  of  the 
kind  of  soil  that  is  to  be  expected  in  any  district,  and  if  along 
with  it  we  study  a  topographical  map,  showing  the  relief  of 
the  surface  and  consequently  what  is  commonly  spoken  of  as 
the  aspect  of  the  farm,  very  useful  information  is  obtained. 
This  last  consideration  is  an  important  one,  and  often  over- 
looked. It  is  obvious  that  of  two  farms  lying  on  similar  rocks, 
but  one  sloping  northwards  and  the  other  south,  the  one  on 

13—2 


196  STKATIGEAPHICAL  GEOLOGY  [OH. 

the  southerly  slope  is  more  likely  to  grow  good  crops  and  to 
rear  healthy  stock.  This  is  for  the  most  part  a  matter  of 
temperature  and  climate,  and  not  really  geological,  but  in  hilly 
and  still  more  in  mountainous  regions  it  is  of  the  greatest 
importance.  Again  exposure  to  or  shelter  from  the  prevailing 
winds  has  great  effect  on  the  character  of  the  crops,  especially 
near  our  eastern  coasts.  Hence  it  appears  that  from  many 
points  of  view  the  study  of  maps  is  of  great  agricultural  im- 
portance. The  dependence  of  water  supply  on  the  underground 
structure  of  the  rocks  has  been  dealt  with  in  an  earlier  chapter 
(see  p.  166),  and  it  was  there  clearly  shown  that  an  acquaintance 
with  the  succession  of  the  stratified  rocks  is  essential  for  a 
successful  attempt  at  water-finding. 

The  rocks  composing  the  accessible  portions  of  the  earth's 
crust  have  been  divided  by  geologists  into  somewhat  arbitrary 
groups,  each  of  which  is  characterized  by  some  special  feature 
distinguishing  it  from  all  others.  The  stratified  rocks  are 
subdivided  partly  by  structural  features  and  lithological 
characters,  but  more  satisfactorily  by  means  of  their  included 
fossils.  Unfortunately  the  oldest  stratified  rocks  do  not  con- 
tain any  fossils,  and  it  is  sometimes  difficult  or  impossible  to 
determine  their  true  sequence  and  the  mutual  relations  of 
detached  portions.  For  this  reason  it  has  been  found  necessary 
to  adopt  an  arbitrary  base-line  and  from  this  to  work  both 
upwards  and  downwards.  The  oldest  definitely  established 
fossils  are  found  in  a  certain  set  of  rocks  which  have  long  been 
called  the  Cambrian  system,  and  the  base  of  this  system  is 
taken  as  the  primary  datum-line;  all  the  rocks  demonstrably 
older  than  this  being  called  Pre-Cambrian.  It  is  claimed  that 
in  America  and  elsewhere  fossils  have  been  found  in  rocks 
older  than  the  Cambrian,  but  even  if  this  be  so,  the  general 
principle  is  not  affected.  The  rocks  both  above  and  below  the 
datum-line  are  divided  into  systems  and  these  again  into  series 
and  even  smaller  units,  as  will  be  hereafter  described.  The 
major  divisions,  the  boundaries  between  the  systems,  are 
frequently  determined  by  the  more  important  unconformities, 
or  breaks  in  the  succession,  while  the  minor  subdivisions  depend 
either  on  lithological  or  on  palaeontological  variations.  Many 


IX] 


STRATIGRAPHICAL  GEOLOGY 


197 


Table  of  the  Stratigraphical  Rock-systems. 


Groups 
Kainozoic  or  Tertiary 


Systems 
fNeogene 
I  Palaeogene 


[Cretaceous 

Mesozoic  or  Secondary    <  Jurassic 

iTriassic 


Palaeozoic  or  Primary 


Permian 

Carboniferous 

Devonian 

Silurian 

Ordovician 

Cambrian 


Pre-Cambrian 


The  systems,  as  defined  in  the  right-hand  column,  are 
divided  into  series  and  these  again  into  stages;  in  very  detailed 
work  even  smaller  subdivisions  are  employed.  The  Pre-Cambrian 
group  includes  an  enormous  thickness  of  rocks,  whose  mutual 
relationships  are  still  uncertain;  several  distinct  systems  are 
recognizable.  As  their  relative  order  of  succession  is  in  some 
cases  unknown  they  are  here  omitted  for  the  sake  of  brevity. 

By  many  writers  the  Tertiary  group  is  cut  up  into  a  larger 
number  of  systems  than  here  shown;  it  is  better  however  to 
regard  these  as  having  the  value  of  series  only,  since  they  are  much 
thinner  than  the  older  systems. 


198  STRATIGRAPHICAL  GEOLOGY  [CH. 

of  these  however  are  of  purely  local  value,  since  the  nature  of 
rocks  of  the  same  age  varies  much  from  place  to  place,  according 
to  conditions  of  formation. 

The  table  on  p.  197  shows  the  principal  divisions  and  sub- 
divisions of  the  stratified  rocks  as  adopted  by  the  majority  of 
British  geologists.  The  classifications  in  use  in  other  countries 
run  on  the  same  general  lines,  and  in  many  instances  the  same 
names  are  employed.  This  branch  of  geology  may  be  said  to 
have  originated  in  this  country  from  the  classical  work  of 
William  Smith,  who  applied  to  the  different  formations  names 
taken  from  the  localities  where  he  had  studied  them.  Hence 
it  results  that  the  names  of  obscure  English  villages,  such  as 
Kimeridge  in  Dorset,  or  even  local  dialect  words,  such  as  Gault, 
have  a  world-wide  currency  and  have  been  adopted  into  most 
civilized  languages.  In  our  colonies  and  in  many  foreign 
countries  a  mistaken  patriotism  has  led  to  the  introduction  of 
many  other  names,  often  equivalent  to  the  original  English 
terms  and  of  no  more  general  application,  so  that  the  whole 
subject  has  been  involved  in  great  confusion.  The  English 
names,  though  often  unsuitable,  have  at  any  rate  the  claim  of 
priority,  while  the  innumerable  foreign  innovations  are  in  many 
cases  entirely  unjustified  and  unnecessary.  The  completeness 
of  the  geological  record  in  the  British  Isles  is  indeed  very 
remarkable;  it  has  been  found  in  only  a  very  few  instances 
that  formations  recognizable  in  other  countries  cannot  be 
identified  in  Britain,  though  often  with  a  somewhat  different 
character. 

It  is  frequently  found  that  a  formation  of  a  given  age,  when 
traced  from  place  to  place  for  a  considerable  distance,  undergoes 
a  gradual  change  in  character,  corresponding  to  an  actual 
difference  of  physical  conditions  at  the  time  of  its  formation. 
The  contemporaneity  of  stratified  rocks  in  two  distant  regions 
can  be  established  in  two  different  ways ;  it  may  be  possible  to 
trace  the  bed  continuously  from  one  place  to  the  other,  though 
this  is  rarely  practicable;  or  the  equivalence  of  age  may  be 
established  by  examination  of  the  contained  fossils,  a  much 
more  convenient  and  certain  method.  If  the  bed  in  question 
itself  contains  no  fossils,  it  may  be  possible  to  establish  its 


ix]  STKATIGKAPHICAL  GEOLOGY  199 

identity  by  noting  its  relation  to  other  strata  of  known  age, 
and  this  is  a  method  of  wide  application. 

The  principle  that  a  stratified  rock  varies  from  place  to 
place  in  harmony  with  the  local  conditions  is  most  important r 
and  has  led  to  the  development  of  the  idea  of  fades,  one  of  the 
most  fruitful  conceptions  in  modern  geology.  The  facies  of 
a  formation  may  be  defined  as  the  sum  of  its  lithological  and 
palaeontological  characters  at  any  given  place.  These  char- 
acters, when  carefully  studied  and  correctly  interpreted,  indicate 
the  physical  conditions  under  which  the  bed  was  formed,  and 
thus  afford  valuable  information  as  to  the  climatic  and 
geographical  conditions  of  past  times.  This  subject  is  not  of 
much  direct  interest  to  the  agriculturist;  but  the  study  of 
facies,  or  lateral  variation  of  strata,  as  it  may  be  called,  has  an 
indirect  interest,  in  the  following  way.  It  is  generally  the 
custom  in  geological  maps  to  indicate  all  the  strata  of  the  same 
age  by  the  same  colour,  irrespective  of  variation  in  character, 
and  it  is  consequently  unsafe  to  assume  that  the  same  colour  in 
two  distant  parts  of  a  map  necessarily  indicates  the  same  kind 
of  rock,  although  it  does  indicate  rock  of  the  same  age.  Many 
instances  of  this  principle  will  appear  in  the  following  chapters. 

Even  within  the  narrow  limits  of  the  British  Isles  many  of 
the  rock-formations  show  a  wide  variation  of  character,  and 
therefore  of  agricultural  value,  when  traced  from  place  to  place, 
and  this  initial  difference  of  composition  is  often  made  still 
more  notable  from  the  point  of  view  of  soil-formation  by  the 
wide  range  of  climatic  conditions  that  here  prevail.  Some 
formations  are  of  such  a  character  that  they  may  yield  good 
soils  in  a  favourable  climate,  and  bad  soils  where  the  conditions 
are  adverse.  In  other  instances  the  inherent  characteristics 
of  the  rock  are  dominant  over  climatic  disabilities  and  the  soil 
may  be  conspicuously  fertile  or  infertile  under  very  varying 
circumstances.  A  good  instance  is  the  Old  Red  Sandstone, 
which  yields  rich  soils,  growing  heavy  crops  even  in  the  inhos- 
pitable climate  of  the  extreme  north  of  Scotland,  as  well  as  in 
the  warm  and  sheltered  Severn  valley.  This  is  an  extreme 
case,  but  other  less  striking  instances  will  appear  later. 


CHAPTER   X 

THE   PRECAMBRIAN  AND   LOWER   PALAEOZOIC   SYSTEMS 

I.     THE  PRECAMBRIAN  SYSTEMS 

The  Precambrian  rocks  of  the  British  Isles  are  almost 
confined  to  the  Highlands  and  Islands  of  Scotland  and  to  the 
north  and  west  of  Ireland.  In  these  two  countries  they  form 
a  large  extent  of  land,  but  in  England  and  Wales  the  areas 
over  which  these  rocks  come  to  the  surface  are  very  limited, 
and  of  little  or  no  agricultural  importance. 

Within  the  limits  of  the  British  Isles  three  very  well- 
marked  and  distinctive  types  of  Precambrian  rocks  have  been 
observed,  as  follows:  (a)  gneisses  and  schists,  (b)  sediments, 
(c)  volcanic  rocks.  It  is  quite  certain  that  the  gneisses  and 
schists  are  the  oldest  of  these  groups,  but  the  relative  ages  of 
the  sediments  and  volcanics  are  uncertain.  If  a  line  be  drawn 
across  Scotland  from  Stonehaven  on  the  east  coast  to  Helens- 
burgh  on  the  Clyde  and  continued  to  the  neighbourhood  of 
Galway  in  Ireland,  this  line  may  be  taken  in  a  general  way  to 
mark  the  southern  boundary  of  the  visible  gneissose  and 
schistose  rocks,  though  even  over  considerable  parts  of  this 
area,  and  especially  in  Ireland,  they  are  covered  by  more  recent 
formations.  Similar  rocks  reappear  over  a  small  area  in 
Anglesey  and  a  still  smaller  patch  exists  in  the  Malvern  Hills. 
The  sedimentary  type  occurs  as  a  narrow  and  often  discon- 
tinuous strip  down  the  west  of  Scotland  from  Cape  Wrath  to 
Islay,  reappearing  again  in  Shropshire,  while  the  volcanic  type 
is  found  in  disconnected  patches  in  Wales,  Shropshire  and 
Leicestershire. 


CH.  x]  LOWER  PALAEOZOIC  SYSTEMS  201 

The  Highland  gneisses  and  schists  include  a  great  variety 
of  rock-types  of  differing  origin,  some  originally  igneous  and 
some  originally  sedimentary,  but  all  agreeing  in  the  fact  that 
they  have  undergone  intense  pressure-metamorphism,  whereby 
they  have  been  wholly  recrystallized  and  new  minerals  formed, 
thus  completely  changing  their  primitive  character.  The  oldest 
rocks  of  all,  forming  the  islands  of  the  Outer  Hebrides  and  a 
narrow  strip  on  the  adjoining  mainland,  are  mainly  igneous  in 
origin.  These  are  called  the  Lewisian  gneisses,  after  the  island 
of  Lewis.  They  consist  for  the  most  part  of  granites,  diorites 
and  other  more  basic  rocks,  intensely  sheared  and  traversed  by 
hundreds  of  dykes,  the  latter  being  largely  converted  into 
hornblende  schists.  Only  near  Loch  Maree  are  there  some 
traces  of  sediments,  including  crystalline  marbles.  The  general 
character  of  the  scenery  may  be  gathered  from  the  following 
description  of  the  gneiss  country  as  seen  on  the  mainland, 
along  the  sea-board  of  Sutherland  and  Ross.  "Throughout 
this  belt  of  country  bare  rounded  domes  and  ridges  of  rock, 
with  intervening  hollows,  follow  each  other  in  endless  succession, 
forming  a  singularly  sterile  tract,  where  the  naked  rock  is 
but  little  concealed  under  superficial  deposits,  and  where  the 
surface  is  dotted  over  with  innumerable  la'kes  and  tarns1."  It 
is  obvious  that  such  country  can  have  little  or  no  agricultural 
value,  and  it  is  mainly  deer-forest. 

Resting  unconformably  on  a  very  uneven  surface  of  Lewisian 
gneiss  is  a  vast  thickness  of  true  sedimentary  rocks,  known  as 
the  Torridonian  or  Torridon  Sandstone  group.  This  consists 
mainly  of  red  felspathic  sandstones  and  conglomerates,  with 
occasional  bands  of  shale,  showing  a  strong  superficial  likeness 
to  the  Old  Red  Sandstone.  Its  Precambrian  age  is  however 
conclusively  proved  by  the  fact  that  in  its  turn  it  is  overlain 
unconformably  by  fossiliferous  Cambrian  strata.  The  Torridon 
sandstone  area  is  in  the  main  mountainous,  and  it  forms  some 
of  the  most  remarkable  peaks  in  the  British  Isles.  Judging 
from  its  general  character  this  formation  would  under  favourable 


1  "The  Geological  Structure  of  the  North- West  Highlands  of  Scotland, 
Mem.  GeoL  Survey,  Glasgow,  1907,  p.  2. 


202  THE   PRECAMBRIAN  AND  [CH. 

conditions  yield  a  rich  soil,  but  the  climate  and  situation  are 
so  adverse  that  this  also  is  in  the  main  under  deer  forests. 

As  previously  mentioned  a  very  similar  rock  reappears  in 
Shropshire,  forming  the  high-lying  district  known  as  the 
Longmynd.  This  reaches  a  height  of  1700  feet  and  is  some 
12  miles  long  by  5  miles  wide.  It  is  mainly  open  unenclosed 
ground,  covered  with  heather  and  devoted  to  sheep-farming, 
the  chief  breeds  being  the  Welsh  Mountain,  Clun  and  Kerry. 
There  are  also  droves  of  more  or  less  wild  ponies.  Only  in  the 
valleys  is  there  a  little  arable  land  with  a  poor,  sour  soil. 

The  greater  part  of  Scotland,  north  of  the  line  as  previously 
denned  (p.  200),  is  occupied  by  a  series  of  rocks  which  may 
be  conveniently  known  collectively  as  the  Highland  Schists; 
these  undoubtedly  include  in  places  patches  of  Lewisian  gneiss, 
brought  up  by  faults,  and  they  are  also  largely  penetrated  by 
great  granite  intrusions,  as  in  Aberdeenshire  and  near  Oban. 
But  the  greater  part  of  the  area  is  occupied  by  a  series  of  rocks, 
originally  sediments  and  now  intensely  metamorphosed,  so 
that  they  have  taken  on  the  structure  of  typical  crystalline 
schists.  Besides  felspathic  gneisses  and  mica-schists  they 
include  also  quartzites,  slates,  and  crystalline  limestones  or 
marbles. 

The  more  highly  felspathic  gneissose  rocks  of  Sutherland 
and  Ross  are  often  spoken  of  as  the  Moine  gneisses,  while  the 
more  definitely  sedimentary  rocks  of  the  central  and  southern 
Highlands  and  of  the  north  of  Ireland  are  called  the  Dalradian 
series.  From  recent  investigations  it  appears  that  the  structure 
of  this  region  is  of  almost  incredible  complexity  and  the  true 
relations  of  the  rock-groups  are  not  as  yet  understood.  However 
it  is  clear  that  the  rocks  are  generally  speaking  thrown  into 
folds  striking  from  N.E.  to  S.W.  and  some  beds,  especially  a 
coarse  conglomerate,  have  been  traced  across  Scotland  from 
sea  to  sea.  The  limestones  and  quartzites  appear  again  in 
force  in  Ireland. 

Since  both  the  rocks  themselves  and  the  climate  show  very 
wide  variations  within  this  area  the  agricultural  characters  of 
this  formation  vary  enormously.  The  western  parts  are  as  a 
rule  distinctly  less  fertile  than  the  eastern  areas  in  the  same 


x]  LOWEK  PALAEOZOIC  SYSTEMS  203 

latitude ;  this  is  due  partly  to  superior  elevation  and  partly  to 
excessive  rainfall.  The  greater  part  of  the  land  is  mountainous, 
being  largely  deer-forests,  grouse-moors  and  sheep-runs,  but  in 
all  the  valleys  and  along  the  east  coast  is  much  fertile  land, 
though  much  of  this  is  certainly  on  glacial  drift  and  other 
superficial  deposits.  In  the  glens  and  sheltered  places  artificial 
plantations  are  successful,  though  in  the  Highlands  natural 
forest  is  almost  non-existent,  and  contrary  to  a  wide-spread 
opinion,  deer-forests  do  not  contain  any  trees.  Especially  in 
the  west,  peat  bogs  are  very  abundant.  Although  there  is 
much  successful  arable  farming,  as  for  example  in  Aberdeen- 
shire,  nevertheless  the  staple  industry  of  the  Highlands  is 
certainly  sheep-grazing,  and  the  rearing  of  the  well-known 
Highland  cattle,  which  are  well  suited  to  a  rigorous  climate  and 
poor  food. 

In  Ireland  the  Dalradian  rocks  occupy  a  large  area,  generally 
at  a  less  average  elevation  than  in  Scotland.  They  are  much 
covered  by  peat  and  other  superficial  deposits,  but  where 
exposed  at  the  surface  they  sometimes  form  what  in  Ireland  is 
considered  fairly  good  land.  Some  parts  however,  as  in  western 
Donegal  and  in  Connaught,  are  very  infertile,  the  climate 
making  any  real  improvement  impossible. 

The  Precambrian  volcanic  rocks  of  Wales  and  of  Shropshire 
cover  so  small  an  area  that  no  description  is  necessary. 
Charnwood  Forest  in  Leicestershire  consists  of  similar  rocks, 
while  the  Malvern  gneisses  also  cover  very  little  ground.  The 
serpentine  and  other  igneous  rocks  of  the  Lizard  in  Cornwall, 
which  are  probably  Precambrian,  form  a  barren  wind-swept 
plateau,  partly  covered  with  heather  and  gorse  and  bearing 
many  rare  plants. 

In  many  other  parts  of  the  world  Precambrian  rocks,  both 
crystalline  schists  and  sediments,  cover  enormous  areas,  and 
yield  soils  of  all  kinds,  their  character  depending  largely  on 
climatic  conditions.  The  Scandinavian  peninsula  is  very 
like  the  Highlands  of  Scotland  on  a  larger  scale;  among 
numerous  examples  special  mention  may  be  made  of  Canada, 
South  Africa  and  India  as  countries  where  the  Precambrian 
rocks  play  a  very  large  part  in  the  structure  of  the  land. 


204  THE  PRECAMBRIAN  AND  [CH. 

Where  conditions  are  favourable  the  soils  yielded  by  them  are 
often  very  fertile.  Hence  the  comparative  barrenness  of  these 
formations  in  Britain  is  clearly  in  the  main  a  matter  of  climate 
and  elevation  rather  than  any  peculiarity  inherent  in  the  rocks 
themselves.  Since  gneissose  and  schistose  rocks  often  possess  a 
composition  very  similar  to  that  of  granites  and  other  igneous 
rocks,  they  must  be  rich  in  many  of  the  elements  of  plant  food, 
only  requiring  favourable  weathering  conditions  to  yield  rich 
soils.  In  many  tropical  regions  such  conditions  exist  to  a  very 
large  extent.  Hence  arises  the  great  fertility  of  Brazil  and 
parts  of  central  Africa,  among  many  other  examples. 

II.     THE  LOWER  PALAEOZOIC  SYSTEMS 

The  Lower  Palaeozoic  systems  of  the  British  Isles  comprise 
an  enormous  thickness  of  rocks  showing  a  general  uniformity  of 
character,  though  there  is  much  variation  in  local  detail.  For 
the  most  part  they  form  elevated  regions  in  the  northern  and 
western  part  of  the  country,  the  three  principal  areas  being 
Wales,  the  Lake  District  and  the  Southern  Uplands  of  Scotland. 
Since  the  total  thickness  is  so  great,  they  are  divided  by 
geologists  into  three  systems,  the  Cambrian,  Ordovician  and 
Silurian.  Unfortunately  there  exists  considerable  confusion  of 
nomenclature,  since  by  many  writers  the  Silurian  system  is 
held  to  include  the  Ordovician,  under  the  name  of  Lower 
Silurian,  while  the  Silurian  in  the  modern  sense  is  called  Upper 
Silurian.  It  is  unnecessary  to  enter  into  this  controversy;  it 
must  suffice  to  say  that  the  old  arrangement  is  now  obsolete, 
and  the  Ordovician  system  is  recognized  by  nearly  all  modern 
authors. 

In  general  terms  it  may  be  stated  that  in  the  main  the 
Lower  Palaeozoic  rocks  consist  of  slates,  grits  and  conglomerates 
with  only  occasional  limestone  bands,  though  these  are  often 
of  some  local  importance.  In  places  also  there  is  an  extensive 
development  of  igneous  rocks  of  Ordovician  age,  as  in  North 
Wales  and  the  Lake  District.  These  volcanic  rocks  form  some 
of  the  wildest  mountain  scenery  in  the  British  Isles,  and  only 
a  very  small  proportion  of  the  whole  series  is  suited  for  high- 
class  farming. 


x]  LOWER  PALAEOZOIC  SYSTEMS  205 

A.    The  Cambrian  System 

The  Cambrian  rocks,  as  the  name  implies,  are  most  exten- 
sively developed  in  Wales,  forming  several  detached  areas  in 
that  country.  The  largest  of  these  are  in  Caernarvonshire  and 
Merioneth. 

The  subdivisions  of  the  Cambrian  system  adopted  in  North 
Wales  are  as  follows: 

Tremadoc  Slates, 

Lingula  Flags, 

Menevian  series, 

Harlech  Grits  and  Llanberis  Slates. 

The  Harlech  Grits  form  a  high,  generally  uncultivated,  area 
in  the  west  of  Merioneth ;  the  soils  are  generally  unproductive 
and  most  of  the  land  is  in  mountain  pasture.  Of  the  same  age 
are  the  well-known  roofing  slates  of  Llanberis,  which  give  rise 
to  an  important  quarrying  industry  at  Bethesda  and  other 
places  in  Caernarvonshire.  The  Menevian  strata,  which  are 
thin  and  much  softer,  generally  come  at  the  bottoms  of  deep 
valleys,  but  the  Lingula  Flags  form  a  wide  extent  of  upland 
country,  much  covered  by  drift  and  peat.  The  Tremadoc 
Slates  are  also  unimportant,  though  yielding  an  inferior  grade 
of  slate. 

The  Cambrian  rocks  of  South  Wales,  which  are  closely 
comparable  with  those  described,  form  some  small  patches  near 
the  city  of  St  Davids  in  Pembrokeshire;  there  are  also  small 
outcrops  in  Shropshire,  in  the  Malvern  Hills,  and  near  Nuneaton, 
in  Warwickshire.  The  Hartshill  quartzite  at  Nuneaton  yields 
a  valuable  road-making  material  much  used  in  the  midland 
counties.  In  the  north-west  of  Scotland  there  is  a  long  strip 
of  Cambrian  rocks,  extending  near  to  the  western  sea-board 
from  the  neighbourhood  of  Loch  Eireboll  to  Skye.  This 
includes  quartzites  and  limestones  or  dolomites,  some  of  the 
latter  being  metamorphosed  to  marble. 

The  Cambrian  rocks  contain  the  earliest  known  fossils,  the 
most  important  group  in  this  system  being  the  trilobites. 
Olenellus,  Pamdoxides,  Olenus,  Asaphus  and  Angelina  are  the 
most  characteristic  genera.  Brachiopods  also  occur,  and  the 


206  THE   PKECAMBRIAN  AND  [OH. 

earliest  graptolites  are  found  in  the  Tremadoc  division. 
However  fossils  are  scarce  and  often  difficult  to  find,  owing  to 
cleavage. 

Since  the  Cambrian  rocks  cover  only  very  small  areas  in 
remote  and  often  mountainous  regions  it  is  unnecessary  to  give 
any  detailed  account  of  their  lithological  character.  Where 
cultivated  soils  are  found  on  Cambrian  rocks  the  subsoil  is 
usually  found  to  be  drift  and  other  superficial  deposits. 

B.    The  Ordovician  System 

The  strata  of  the  Ordovician  system  cover  large  areas  in 
Wales,  the  Lake  District,  and  the  south  of  Scotland.  They 
form  for  the  most  part  elevated  ground,  often  indeed  rising 
into  mountains.  This  system  differs  from  the  Cambrian  in 
that  it  includes  in  North  Wales  and  the  Lake  District  great 
thicknesses  of  volcanic  rocks,  both  ashes  and  lavas,  but  the 
sediments  are  of  much  the  same  general  character  as  the 
Cambrian,  being  mainly  slates  and  grits,  with  only  local  cal- 
careous developments  of  no  great  thickness. 

The  rocks  of  the  Ordovician  system  are  divided  into  four 
groups,  as  follows: 

Ashgillian  series, 

Caradocian  series, 

Llandeilo  series, 

Arenig  or  Skiddavian  series. 

The  Caradocian  and  Ashgillian  together  are  equivalent  to 
the  older  designation  of  Bala  series ;  the  latter  name  has  now 
been  generally  abandoned,  owing  to  a  want  of  precision  in 
definition.  The  typical  development  of  the  system  is  seen  in 
Merioneth  and  Caernarvon,  though  the  rocks  of  Shropshire  and 
the  Lake  District  are  perhaps  equally  important  geologically, 
if  not  agriculturally. 

In  North  Wales  the  Arenig  rocks  rest  with  a  slight  uncon- 
formity on  the  Cambrian ;  at  the  base  is  a  conglomerate,  with 
shales  or  slates  above.  In  the  Arenig  mountains  and  near 
Dolgelly  is  a  great  thickness  of  lavas  and  ashes,  mostly  forming 
very  wild  uncultivated  ground.  The  Llandeilo  series  is  also 


x]  LOWER  PALAEOZOIC  SYSTEMS  207 

in  the  main  slates,  with  some  igneous  rocks,  but  the  two  upper 
subdivisions  show  more  variety,  being  in  some  places  an 
alternation  of  slates  and  limestones,  in  others  being  represented 
l>y  the  great  lavas  and  ashes  of  the  Snowdon  district.  Snowdon 
itself  is  mainly  composed  of  lava  flows. 

The  Ordovician  rocks  of  Pembrokeshire  are  closely  com- 
parable with  those  of  North  Wales,  though  igneous  rocks  are 
less  conspicuous.  In  most  geological  maps  almost  the  whole 
of  central  Wales  is  indicated  as  being  composed  of  Ordovician 
rocks,  but  in  reality  a  good  deal  of  this  area  is  occupied  by 
Silurian  strata,  as  shown  by  the  recent  detailed  study  of  some 
portions.  However  the  two  systems  are  here  very  similar,  both 
lithologically  and  agriculturally.  Ordovician  rocks  also  occur 
to  a  considerable  extent  in  Shropshire,  both  east  and  west  of 
the  Longmynd.  Here  they  form  rather  poor  soils,  mostly 
heavy  and  wet,  those  on  the  east  being  better  than  those  on 
the  west. 

The  Ordovician  rocks  of  the  Lake  District  may  also  be 
divided  into  four  groups,  correlated  with  those  of  Wales ;  they 
are  as  follows: 

The  Ashgill  Shale, 

The  Coniston  Limestone, 

The  Borrowdale  Volcanic  series, 

The  Skiddaw  Slates. 

The  Skiddaw  Slates  include  a  great  thickness  of  rather  soft 
slates,  generally  of  poor  quality,  with  some  grits.  However  it 
is  known  that  part  of  the  series  is  Cambrian,  there  being  here 
no  visible  break  in  the  succession.  The  Skiddaw  Slates  form 
all  the  northern  part  of  the  Lake  District,  including  the  Skiddaw- 
Saddleback  mountain  mass  and  they  extend  far  to  the  west, 
even  beyond  Ennerdale.  The  higher  parts  are  almost  entirely 
open  sheep  ground ;  in  the  valleys  there  is  much  drift  and  peat, 
and  the  soil  is  on  the  whole  poor.  In  the  north  the  lower  ground 
is  mainly  covered  with  boulder-clay  and  the  soils  are  very  stony. 
Besides  the  slate  quarries  there  are  lead  mines,  now  largely 
abandoned,  and  the  iron-mining  district  of  west  Cumberland 
extends  into  the  Skiddaw  Slate  area  east  of  Cleator  Moor. 


208  THE  PRECAMBRIAN  AND  [CH. 

The  Borrowdale  Volcanic  series,  which  is  computed  to  have 
a  total  thickness  of  some  30,000  feet,  forms  -the  main  part  of 
the  Lake  District  mountains,  including  the  Scafell  group  and 
the  Helvellyn  range.  Most  of  the  country  is  exceedingly  steep 
and  rocky;  only  in  a  few  valleys  is  there  any  cultivation, 
mostly  on  drift  and  alluvium,  and  the  highest  summits  are  bare 
rock.  Sheep-farming  is  almost  the  only  kind  of  agriculture. 
The  chief  economic  products  are  the  famous  green  slates 
(cleaved  volcanic  ashes)  and  the  graphite  of  Borrowdale,  used 
for  lead-pencils. 

The  Coniston  Limestone  and  the  Ashgill  Shales  are  both 
thin  and  their  outcrop,  though  traceable  for  many  miles  from 
Shap  towards  the  south-west,  is  too  narrow  to  be  of  any 
importance.  There  is  also  a  narrow  strip  of  Ordovician  rocks 
in  the  Eden  valley,  along  the  foot  of  the  Cross  Fell  range ;  this 
is  largely  open  sheep  ground. 

The  Ordovician  system  of  the  Lake  District  is  the  home 
of  the  Herdwick  breed  of  sheep;  apart  from  sheep-farming 
agriculture  is  not  in  a  flourishing  condition,  largely  on  account 
of  the  excessive  rainfall,  and  the  mountainous  nature  of  the 
ground. 

Ordovician  rocks  also  occur  to  a  considerable  extent  in  the 
Southern  Uplands  of  Scotland,  but  they  are  there  inextricably 
mixed  up  with  the  Silurian,  both  formations  showing  similar 
lithological  characters  in  the  same  areas,  though  varying  much 
from  place  to  place ;  hence  consideration  of  this  region  may  well 
be  deferred  till  the  Silurian  system  is  dealt  with. 

The  Ordovician  rocks  are  often  fossiliferous,  except  of 
course  where  they  are  of  volcanic  origin.  The  fauna  is  fairly 
large,  trilobites  being  still  abundant,  though  the  graptolites 
are  perhaps  even  more  important.  By  means  of  the  latter  group 
it  has  been  found  possible  to  divide  the  whole  system  into  a 
series  of  minor  subdivisions  called  zones,  traceable  in  all  the 
different  areas,  the  same  forms  occurring  in  the  same  succession 
also  in  Scandinavia,  North  America  and  other  parts  of  the  world. 
The  most  important  Ordovician  genera  are  Dichogmptus, 
Tetragraptus,  Phyllograptus,  Didymograptus,  Dicellograptus, 
Dicranograptus,  Diplograptus  and  Climacograptus.  Among  the 


x]  LOWER  PALAEOZOIC    SYSTEMS  209 

trilobites  the  most  characteristic  are  Asaphus,  Ampyx,  Ogygia, 
Phacops  and  Trinucleus.  Among  the  brachiopods  are  Orthis, 
Slrophoirnena  and  Leptaena.  Corals  and  the  very  remarkable 
cystideans  are  common  in  the  limestones. 


C.    The  Silurian  System 

As  previously  explained  the  term  Silurian  is  here  used  in 
the  modern  restricted  sense,  equivalent  to  the  Gothlandian  of 
some  recent  continental  writers.  This  includes  only  the  Upper 
Silurian  of  the  earlier  British  authors.  In  most  parts  of  the 
British  Isles,  where  Silurian  rocks  are  seen  at  the  surface,  they 
present  a  facies  very  similar  to  that  of  the  preceding  Ordovician 
system,  namely,  a  great  thickness  of  shales  and  grits  of  very 
monotonous  character ;  it  is  only  in  Shropshire  and  the  Welsh 
borderland  that  limestones  are  developed  to  any  extent.  It  is 
an  unfortunate  fact  that  in  Shropshire,  usually  regarded  as  the 
type  area,  the  development  of  the  system  is  abnormal,  with 
thick  beds  of  limestone.  Everywhere  else  the  gritty  and  shaly 
facies  is  dominant.  The  names  applied  to  the  principal  sub- 
divisions are  largely  taken  from  the  Shropshire  district,  conse- 
quently in  most  parts  of  the  country  they  are  entirely 
inapplicable. 

The  Silurian  rocks  crop  out  at  the  surface  over  considerable 
areas  in  Shropshire  and  Herefordshire,  and  form  a  large  part 
of  Wales.  As  already  mentioned,  they  have  not  yet  been 
properly  demarcated  from  the  Ordovician  in  central  Wales. 
They  also  occupy  large  areas  in  the  southern  part  of  the  Lake 
District  and  the  adjoining  Howgill  Fells,  and  in  the  Southern 
Uplands  of  Scotland.  In  Ireland  also  there  are  many  scattered 
patches. 

Two  distinct  classifications  of  the  strata  are  in  general  use, 
as  shown  side  by  side  in  the  following  table: 

Downton  and  Led  bury  beds     )      ^ 

TT  Downtoman  series. 

Upper  Ludlow J 

Lower  Ludlow  ...  . .  1 

Wenlock  series  /     Salopian  series. 

Tarannon  Slates  ..) 

T  T    '  j  i    j  Valentian  series. 

Llandovery  beds          J 

R.  A  G.  14 


210  THE  PRECAMBRIAN  AND  [CH. 

The  classical  district  for  the  Silurian  succession  is  in  Shrop- 
shire, in  the  country  between  Much  Wenlock  and  Ludlow. 
Here  the  system  rests  unconformably  on  the  Ordovician,  the 
Lower  Llandovery  being  absent.  The  Upper  Llandovery  is 
represented  by  the  May  Hill  Sandstone,  a  calcareous  sandstone 
with  many  fossils,  especially  StricTdandinia  and  Pentamerus 
oblongus.  The  Tarannon  series,  if  present  at  all,  is  unimportant. 
At  the  base  of  the  Wenlock  series  in  some  places  comes  the 
Woolhope  limestone,  which  is  inconstant,  occurring  here  and 
there  in  lenticular  masses.  The  greater  part  of  the  Wenlock 
series  consists  of  a  thick  mass  of  black  shales  with  graptolites 
of  the  Monograptus  priodon  group.  At  the  top  comes  the  well- 
known  Wenlock  Limestone,  economically  the  most  important 
bed  of  the  whole  system.  This  is  largely  quarried  for  lime- 
burning  and  for  use  in  the  iron-furnaces  of  the  Black  Country. 
It  is  a  grey  limestone,  about  100  feet  thick  and  extraordinarily 
fossiliferous,  showing  many  of  the  characters  of  a  coral-reef. 
Corals,  crinoids,  brachiopods  and  trilo bites  are  very  abundant 
and  well  preserved.  Some  important  forms  are  Favosites, 
Heliolites,  Halysites,  Acervularia,  Pentamerus  galeatus,  Atrypa 
reticularis,  Calymene  Blumenbachii,  Phacops  caudatus,  Encri- 
nurus  punctatus. 

The  Lower  and  Upper  Ludlow  beds  are  lithologically  very 
similar;  the  Lower  Ludlow  contains  graptolites  of  the  Mono- 
graptus colonus  type,  but  this  group  became  extinct  at  the  top 
of  this  subdivision.  The  dividing  line  between  the  Lower  and 
Upper  subdivisions  is  formed  by  the  Aymestry  Limestone, 
characterized  by  Pentamerus  Knightii.  The  Upper  Ludlow 
consists  of  grey  flaggy  shales  with  Chonetes  striatella  and  a 
so-called  "  bone  bed,"  full  of  bones  and  spines  of  fish  and  other 
marine  animals.  The  Downton  beds  are  sandstones,  gradually 
becoming  more  and  more  like  the  overlying  Old  Red  Sandstone. 
The  principal  fossils  are  Eurypterids. 

In  North  and  Central  Wales  certain  changes  are  seen  in 
the  lithological  characters  of  the  system,  the  Llandovery  and 
especially  the  Tarannon  series  becoming  much  thicker;  near 
Plynlimmon  the  latter  attains  about  3000  feet.  The  limestones 
disappear  and  the  whole  system  becomes  a  very  thick  and 


x]  LOWER  PALAEOZOIC  SYSTEMS  211 

monotonous  succession  of  shales  and  massive  grits.  In  central 
Wales  numerous  graptolite  zones  have  been  established,  and 
most  of  them  recur  also  in  the  Lake  District  and  Scotland. 

In  the  Lake  District  the  Valentian  series  is  represented  by 
thin  black  shales  with  graptolites,  the  Stockdale  shales,  the  Wen- 
lock  is  flaggy,  and  the  Ludlow  series  expands  to  an  enormous 
thickness,  estimated  at  14,000  feet,  forming  a  great  expanse  of 
poor  high-lying  ground,  both  in  the  southern  part  of  the  Lake 
District  and  in  the  Howgill  Fells,  as  far  east  as  Sedbergh.  In 
Scotland,  near  Moffat,  the  Llandovery  division  alone  consists  of 
black  graptolitic  shale,  all  the  rest,  so  far  as  it  is  present,  being 
thick  grits  with  occasional  shale  bands  (Gala  grits  and  Riccarton 
series).  These  form  most  of  the  open  high  ground  of  the 
Southern  Uplands  and  are  many  thousands  of  feet  thick. 
Throughout  this  region  the  upper  limit  of  the  Silurian  is  some- 
what uncertain,  and  only  at  Lesmahagow  in  Lanark  is  there  any 
representative  of  the  red  Downton  sandstone.  The  Silurian 
rocks  of  County  Down  are  much  like  those  of  the  Southern 
Uplands ;  in  the  rest  of  Ireland  they  are  mainly  covered  by 
peat  and  drift. 

Throughout  the  greater  part  of  all  these  areas  the  Silurian 
rocks  form  high  ground,  which  is  still  largely  unenclosed  and 
devoted  to  sheep-farming.  In  the  valleys,  where  the  soil  is 
sometimes  fairly  fertile  and  well  cultivated,  it  lies  for  the  most 
part  on  drift  and  is  not  to  be  regarded  as  derived  directly  from 
Silurian  rocks,  though  these  undoubtedly  in  most  cases  supplied 
the  greater  part  of  the  material.  Agriculturally  the  Shropshire 
area  differs  a  good  deal  from  most  of  the  others,  being  generally 
at  a  lower  elevation  and  somewhat  more  fertile,  partly,  no  doubt, 
owing  to  the  mixed  nature  of  the  soils  and  the  presence  of  much 
rainwash.  The  occurrence  of  numerous  limestone  beds  also 
has  an  ameliorating  influence  on  the  soils.  In  many  places  a 
good  deal  of  material  has  been  washed  down  from  Old  Red 
Sandstone  outcrops  at  higher  levels,  and,  as  will  appear  in  the 
next  chapter,  this  formation  yields  soils  of  remarkable  fertility. 
The  Wenlock  limestone  forms  steep  ridges  covered  by  woods, 
but  the  Wenlock  shales  form  a  large  expanse  of  land  of  somewhat 
variable  character ;  some  parts  are  heavy  clays,  good  for  wheat 

14—2 


212  THE   PKECAMBRIAN  AND  [CH. 

and  beans.  Taken  as  a  whole,  this  formation  is  not  very  fertile, 
though  there  is  some  fairly  good  grass.  The  soils  of  the  Ludlow 
beds,  though  still  on  the  heavy  side,  are  more  loamy  than 
those  of  the  Wenlock  shale.  In  the  valleys  is  some  good  grass 
land,  but  the  upland  region  of  Clun  Forest  is  open  sheep  walks, 
with  a  local  breed  of  sheep. 

The  major  part  of  the  Silurian  area  of  central  and  north 
Wales  consists  of  high  mountains  and  tablelands,  with  deep 
valleys  between.  On  the  hills  the  soils  are  largely  peaty  and 
covered  with  poor  grass  or  heather,  devoted  to  sheep.  In  the 
larger  valleys  there  is  a  fair  amount  of  arable  land  of  mediocre 
quality  and  much  coarse  wet  pasture.  The  Silurian  tract  of  the 
Lake  District  covers  a  great  area ;  on  the  whole  it  is  less  elevated 
and  rugged  than  the  part  occupied  by  the  Ordovician  system, 
but  still  high  and  for  the  most  part  unenclosed  ground,  largely 
occupied  by  the  Herdwick  sheep.  The  character  of  the  agricul- 
ture of  Westmorland  can  be  gathered  from  the  fact  that  in 
1907  there  were  only  127  acres  of  wheat  in  the  whole  county, 
while  oats  occupied  nearly  14,000  acres.  The  total  area  of  the 
county  is  505,000  acres,  hence  the  proportion  of  arable  land  is 
very  small. 

The  Ordovician  and  Silurian  rocks  of  the  Southern  Uplands 
of  Scotland  form  a  tract  of  ground  for  the  most  part  at  high 
elevations  and  largely  devoted  to  sheep-farming.  Though  less 
rugged  than  the  Lake  District  the  country  is  yet  largely  uncul- 
tivated, being  covered  with  short  grass  or  with  heather.  Only 
in  the  valleys  of  the  numerous  rivers  is  there  much  arable  land, 
but  here  the  soil  is  often  fertile  and  well-farmed.  Here  again 
however  the  subsoil  is  largely  composed  of  drift  and  alluvium. 
The  sheep-farming  of  the  Southern  Uplands  is  of  great  importance 
and  has  given  rise  to  a  considerable  woollen  industry  at  Gala- 
shiels  and  Selkirk.  Further  west  in  Ayrshire  and  Galloway  the 
country  is  less  elevated  and  forms  a  good  agricultural  district, 
where  dairy-farming  is  largely  carried  on.  The  moist  climate 
here  is  specially  favourable  to  turnips  and  very  heavy  crops 
are  grown. 

Summary.  With  few  and  not  very  important  exceptions 
the  Precambrian  and  Lower  Palaeozoic  rocks  of  the  British 


x]  LOWER   PALAEOZOIC   SYSTEMS  213 

Isles  form  mountainous  and  hilly  ground  in  the  northern  and 
western  parts  of  the  country.  This  disposition  of  the  rocks  is 
a  result  of  the  general  geological  structure  of  the  country.  The 
older  rocks  have  undergone  the  highest  degree  of  metamorphism, 
being  in  consequence  hard  and  resistant;  also  they  have  at 
various  times  been  folded  and  elevated  into  mountain  chains. 
The  numerous  intercalations  of  igneous  rock  are  also  hard  and 
denuded  with  difficulty.  Consequently  the  older  rocks  form 
the  highest  parts  of  the  country.  Again,  climate  has  a  most 
important  influence  in  determining  the  character  of  the  soil  and 
vegetation.  Mainly  as  a  result  of  the  present  relief  of  the  land 
the  climate  of  the  British  Isles  shows  extraordinary  variations 
for  so  small  an  area.  This  is  clearly  manifested  by  the  rainfall 
maps  and  by  the  peculiar  distribution  of  the  isotherms.  The 
peculiarities  of  the  climate  are  of  course  greatly  intensified  by 
the  fact  of  the  high  land  being  in  the  west.  If  the  west  coast 
were  occupied  by  low  land  it  would  naturally  be  warm  and  damp 
owing  to  the  prevailing  Atlantic  winds  bringing  rain.  The 
effect  of  the  high  elevation  of  the  western  side  is  of  course  to 
exaggerate  the  rainfall,  while  the  effect  of  the  higher  temperature 
is  to  a  great  extent  neutralized  by  the  height  of  the  land.  At  the 
sea-coast  and  in  sheltered  valleys  in  the  west  the  climate  is 
surprisingly  mild  and  vegetation  luxuriant,  but  on  the  wind- 
swept tablelands  and  on  exposed  portions  of  the  mountains  the 
land  is  often  almost  barren,  or  only  fit  for  the  growth  of  heather 
and  peat  at  quite  small  elevations.  Again,  the  comparatively 
recent  occurrence  of  glacial  conditions  among  the  mountains  of 
the  British  Isles  has  had  an  important  influence  on  the  soils. 
The  ice  swept  the  weathered  material  from  the  hills,  leaving  in 
many  places  nothing  but  bare  unweathered  rock.  After  the 
departure  of  the  ice,  the  conditions  were  such  as  favoured  the 
accumulation  of  peat  rather  than  a  renewed  formation  of 
fertile  agricultural  soils,  and  in  the  Highlands  of  Scotland,  the 
western  Isles,  some  parts  of  the  Pennine  Chain,  in  Wales  and 
over  a  large  part  of  Ireland,  the  peaty  condition  has  persisted  to 
the  present  day.  Furthermore  the  glaciers  laid  down  in  the 
valleys  and  on  the  low  ground  at  the  foot  of  the  mountains 
vast  accumulations  of  boulder-clay,  sands  and  gravels,  consisting 


214  THE   PKECAMBRIAN  AND  [CH. 

of  material  largely  in  an  unweathered  condition  and  containing 
little  plant  food  in  the  available  state.  Many  of  the  eoils  on 
these  glacial  deposits,  though  potentially  rich,  are  still  somewhat 
raw  and  incomplete,  since  scarcely  sufficient  time  has  elapsed 
since  the  departure  of  the  ice  for  the  processes  of  chemical  and 
bacterial  decomposition  to  produce  their  full  effect. 

Again  it  must  be  admitted  that  the  majority  of  the  rocks 
composing  the  older  formations  were  not  such  as  would,  even 
under  favourable  conditions,  produce  soils  of  first-class  quality. 
The  prevailing  rock-types  are  slates  and  grits,  neither  of  which 
usually  yield  fertile  soils,  even  in  more  recent  formations  in 
better  climates.  There  is  commonly  a  deficiency  of  lime,  and 
phosphoric  acid  may  also  be  scarce.  Potash  is  commonly 
present  in  sufficient  quantity,  but  often  in  forms  not  readily 
available.  As  a  result  of  all  these  causes  working  together,  the 
soils  of  the  older  rocks  are  generally  far  from  fertile  and  agri- 
culture is,  with  few  exceptions,  in  a  backward  state. 

Besides  all  these  natural  factors  there  is  also  another 
influence  to  be  reckoned  with,  namely,  the  character  of  the 
inhabitants  of  these  districts.  The  Highlands  of  Scotland, 
Wales  and  Ireland  are  mainly  inhabited  by  a  race  different 
from  that  dwelling  in  England  and  the  Lowlands  of  Scotland, 
possessing  different  standards  and  ideals.  The  question  is 
largely  an  economic  one.  Till  recently  these  people  were  less 
advanced  in  civilization  than  the  rest  of  the  country;  they 
possessed  little  capital  and  had  made  small  progress  in  the  appli- 
cation of  modern  methods  to  agriculture.  It  is  an  interesting, 
but  perhaps  somewhat  unprofitable  subject  for  discussion,  to 
what  extent  this  backwardness  was  due  to  the  character  of  the 
people  themselves,  and  to  what  extent  to  the  natural  disad- 
vantages of  climate  and  soil  with  which  they  had  to  contend. 
It  may  at  any  rate  be  admitted  that  in  the  latter  respect  they 
were  heavily  handicapped  by  Nature.  The  earlier  formations 
also  are  singularly  lacking  in  products  of  economic  value,  so 
that  there  was  little  incentive  to  migration  from  elsewhere  into 
the  districts  occupied  by  them,  leading  to  a  growth  of  population 
round  any  particular  centres.  The  inhabitants  consequently 
remained  in  the  pastoral  stage  of  civilization,  a  state  of  affairs 


x]  LOWER  PALAEOZOIC  SYSTEMS  215 

still  prevailing  over  the  greater  part  of  the  area.  Of  late  years, 
owing  to  emigration,  the  population  of  certain  large  parts  of 
Ireland  and  Scotland  has  greatly  diminished  and  a  good  deal 
of  land  has  actually  gone  out  of  cultivation. 

From  a  consideration  of  the  facts  thus  briefly  outlined,  it  is 
clear  that  the  ultimate  basis  of  the  social  and  economic  problems 
of  these  regions  is  mainly  geological. 


CHAPTER   XI 

THE  DEVONIAN  AND  CARBONIFEROUS  SYSTEMS 

Each  of  these  systems  covers  a  very  large  area  in  the  British 
Isles  and  both  are  of  great  agricultural  interest;  the  rocks  of 
the  Devonian  system  yield  some  of  the  best  land  in  this  country, 
and  the  Carboniferous,  though  not  naturally  very  fertile,  is 
nevertheless  of  great  practical  importance,  largely  owing  to 
the  coal  contained  in  its  upper  part.  The  coal-fields  are 
naturally  centres  of  crowded  population,  and  agriculture  on 
them  and  in  the  neighbouring  areas  shows  some  special  features. 

I.    THE  DEVONIAN  AND  OLD  RED  SANDSTONE  SYSTEM 

One  of  the  most  remarkable  features  of  this  system  is  the 
occurrence  of  two  different  and  sharply  contrasted  facies, 
varying  widely  from  each  other  in  almost  all  respects.  These 
facies  are  known  as  the  Devonian  and  Old  Red  Sandstone  types 
respectively.  The  Devonian  rocks  in  the  strict  sense  of  the 
term  are  in  this  country  confined  to  the  peninsula  of  Devon  and 
Cornwall ;  the  Old  Red  Sandstone  facies  is  found  in  South  Wales 
and  the  west  of  England,  and  very  largely  in  Scotland,  extending 
also  into  the  north  of  Ireland.  In  the  south-west  of  Ireland 
the  rocks  of  this  age  again  differ  a  good  deal  from  both  types, 
but  show  more  resemblance  to  those  of  North  Devon  than  to  the 
Old  Red  Sandstone.  These  variations  are  to  be  accounted  for 
by  the  occurrence  in  late  Silurian  and  Devonian  times  of  a  great 
series  of  earth-movements,  leading  to  profound  modifications 
in  the  distribution  of  land  and  sea.  The  marine  phase  of  the 
Lower  Palaeozoic  era  came  to  an  end,  and  over  the  greater  part 


CH.  xi]  CARBONIFEROUS  SYSTEMS  217 

of  the  British  Isles  continental  conditions  set  in ;  only  in  Devon- 
shire and  Cornwall  did  open  sea  continue  to  prevail.  This 
great  change  was  heralded  by  the  gradual  appearance  of  more 
and  more  continental  characters  in  the  uppermost  beds  of  the 
Silurian.  The  Down  ton  sandstone  clearly  shows  an  approxi- 
mation to  the  Old  Red  Sandstone  fades,  both  in  its  lithological 
character  and  in  its  fossils.  In  Scotland  volcanic  rocks  are 
abundant  in  the  Old  Red  Sandstone  and  the  earth-movements 
were  accompanied  by  intrusion  of  great  masses  of  granite  in 
northern  England,  Scotland  and  Ireland.  Great  mountain 
ranges  were  formed  from  the  older  rocks,  and  their  denudation 
gave  rise  to  the  thick  accumulations  of  the  Old  Red  Sandstone 
in  the  northern  and  western  parts  of  the  British  Isles. 

A.    The  Marine  Devonian  Facies 

The  effects  of  these  earth-movements  were  not  conspicuous 
south  of  the  line  now  occupied  by  the  Bristol  Channel,  and  in 
Devon  and  Cornwall  marine  conditions  still  prevailed.  From 
the  varying  character  of  the  rocks  in  this  area  it  can  be  shown 
that  the  shore  of  this  sea  lay  not  far  north  of  Devonshire, 
while  in  South  Devon  and  Cornwall  the  water  was  deeper. 
The  open  sea  was  clearly  limited  also  towards  the  east,  since  in 
the  Mendip  Hills  in  Somerset  the  Old  Red  Sandstone  facies  is 
seen.  The  Devonian  rocks  of  north  Devon  and  west  Somerset 
form  a  thick  series  of  marine  and  often  fossiliferous  strata  of 
somewhat  variable  character.  The  base  of  the  series  is  nowhere 
visible  and  it  is  not  known  on  what  they  rest.  The  greater 
part  of  the  succession  consists  of  yellow,  brown  and  reddish 
sandstones  and  grits,  but  there  are  also  important  beds  of  slate 
and  limestone.  In  some  respects  the  sandstones  show  a 
considerable  resemblance  to  the  Old  Red  Sandstone  in  neigh- 
bouring areas,  but  they  are  shown  by  fossil  evidence  to  be  of 
marine  origin.  The  rocks  have  been  divided  into  Lower, 
Middle  and  Upper  series,  but  much  controversy  has  arisen  as 
to  the  true  age  of  a  part  of  the  series  known  as  the  Morte  Slates. 
From  their  apparent  stratigraphical  position  they  should  be 
Middle  Devonian,  but  the  few  and  badly  preserved  fossils 
prove  them  to  be  Lower  Devonian  or  even  older;  it  is 


218  THE  DEVONIAN  AND  [CH. 

now  generally  believed  that  their  present  position  is  due  to 
faulting. 

The  Devonian  rocks  of  the  northern  type  occur  only  in 
north  Devon  and  west  Somerset,  occupying  the  area  bounded 
on  the  north  and  west  by  the  coast,  on  the  south  by  the 
Taunton  and  Barnstaple  railway  to  a  point  about  6  miles  west 
of  Taunton,  the  boundary  then  running  north  and  meeting  the 
coast  again  near  Minehead.  This  area  is  generally  high  ground 
and  includes  the  well-known  Exmoor  Forest,  of  which  the 
highest  point  is  Dunkery  Beacon,  1707  feet.  The  eastern 
portion  forms  the  Brendon  Hills.  The  whole  area  is  notorious 
for  the  steepness  of  its  roads,  and  it  is  famous  also  as  the  home 
of  the  wild  red  deer.  The  highest  ground  is  open  and  unenclosed, 
being  covered  with  heather  and  gorse.  In  the  steep  narrow 
valleys  are  many  woods,  and  the  soils  of  the  cultivated  portions 
are  not  as  a  rule  of  first-class  quality.  The  soils  formed  from 
the  grits  are  often  poor  and  stony,  but  the  bands  of  slate  and 
limestone  are  of  better  quality  and  more  fertile. 

The  Devonian  rocks  of  south  Devon  are  separated  from  the 
area  just  described  by  a  broad  belt  occupied  by  Carboniferous 
rocks,  forming  the  middle  part  of  a  great  syncline.  Below  this 
Carboniferous  belt  the  Devonian  rocks  undergo  a  lithological 
change,  emerging  on  the  southern  side  as  a  series  mainly 
argillaceous  and  calcareous,  with  well-cleaved  slates  and  thick 
massive  limestones.  There  is  some  uncertainty  as  to  the  base 
of  this  series,  and  of  late  years  it  has  been  shown  that  certain 
beds  in  southern  Cornwall  formerly  believed  to  be  Devonian 
are  really  Ordovician  and  perhaps  also  Silurian.  So  far  as  the 
sequence  is  known  the  lower  part  appears  to  be  mainly  com- 
posed of  grits,  while  the  Middle  Devonian  is  more  slaty,  and 
the  Upper  Devonian  mainly  massive  limestones.  In  Cornwall 
almost  the  whole  series  is  slaty,  and  is  generally  known  locally 
as  "killas."  The  Devonian  rocks  have  undergone  intense 
metamorphism,  and  the  structure  of  the  region  is  very  compli- 
cated in  detail,  owing  to  folding,  faulting,  overthrusting  and 
igneous  intrusions. 

The  most  striking  geological  feature  in  Devon  and  Cornwall 
is  the  great  granite  intrusions,  of  post-Carboniferous  age.  The 


xi]  CARBONIFEROUS   SYSTEMS  219 

largest  of  these  is  the  Dartmoor  granite;  this  was  intruded 
along  the  line  of  junction  between  the  Devonian  and  Car- 
boniferous rocks  and  forms  a  somewhat  irregular  mass  about 
20  miles  in  diameter.  Dartmoor  is  a  wild  upland  region, 
mainly  uncultivated  and  covered  with  heather.  There  are 
valuable  granite  quarries  and  an  important  tin-mining  industry ; 
the  latter  however  is  gradually  decreasing  owing  to  exhaustion 
of  the  accessible  ore.  In  Cornwall  also  there  are  four  large 
granite  masses  and  some  smaller  ones.  The  mining  industry 
of  southern  Cornwall  is  still  of  great  importance,  as  there  are 
abundant  veins  containing  tin,  copper,  zinc,  arsenic  and  other 
ores.  The  total  number  of  minerals  found  in  the  Cornish  veins 
is  very  great  and  many  are  of  great  scientific  interest  as  well  as 
of  practical  value.  Among  some  recent  developments  mention 
may  be  made  of  the  exploitation  of  pitchblende  as  a  source  of 
radium.  The  granite  intrusions  have  produced  a  high  grade  of 
metamorphism  in  the  surrounding  sediments  and  have  them- 
selves undergone  some  remarkable  changes  as  a  result  of  the 
action  of  highly  heated  vapours  (pneumatolysis).  These 
changes  are  of  several  kinds,  but  the  most  important  econo- 
mically is  kaolinization,  or  the  conversion  of  the  felspar  of  the 
granite  into  china-clay1.  This  is  the  basis  of  an  industry  of 
great  importance. 

The  relief  of  the  area  occupied  by  Devonian  rocks  is  very 
varied,  but  on  the  whole  the  country  is  composed  of  a  deeply 
dissected  plateau,  with  steep-sided  valleys;  in  these  there  is 
much  woodland.  For  the  most  part  the  soils  on  the  Devonian 
rocks  are  not  naturally  very  fertile,  but  there  is  much  rainwash 
and  alluvium  of  good  quality,  and  there  are  many  prosperous 
agricultural  districts.  In  many  parts  of  Devon  and  Cornwall 
dairy-farming  is  of  much  importance.  The  volcanic  rocks  yield 
the  richest  soils;  the  slaty  members  of  the  Devonian  system 
produce  rather  heavy  clay  soils,  those  on  the  grits  being  some- 
what lighter  in  character.  The  limestone  soils  also  are  for  the 
most  part  rather  thin  and  clayey,  and  consequently  much 
affected  in  seasons  of  drought.  Owing  to  the  mild  climate 

1  Reid  and  Flett,  "The  Geology  of  the  Land's  End  District,"  Mem.  GeoL 
Surv.  1907,  pp.  53-60. 


220  THE   DEVONIAN  AND  [CH. 

many  semitropical  plants  can  be  grown  in  the  open  air  in 
sheltered  valleys  and  near  the  coast,  and  the  growing  of  early 
flowers  and  vegetables  forms  a  very  flourishing  industry. 

B.    The  Old  Red  Sandstone 

The  strata  comprised  under  the  general  name  of  the  Old 
Red  Sandstone  form  a  typical  example  of  a  continental  fades. 
They  include  sandstones,  shales,  marls  and  conglomerates  with 
occasional  limestones;  the  fauna  is  very  scanty  and  almost 
confined  to  fish ;  these  however  are  abundant  in  some  localities. 
In  Scotland  volcanic  rocks  are  largely  developed.  The  exact 
nature  of  the  conditions  of  deposition  of  the  Old  Red  Sandstone 
are  not  known  with  certainty,  but  it  is  at  any  rate  clear  that 
there  were  several  disconnected  areas.  Some  authorities 
maintain  that  these  were  lakes,  while  others  hold  them  to  have 
been  long  narrow  arms  of  the  sea,  lying  between  the  folded 
mountain  chains  of  older  rocks  that  began  to  arise  towards  the 
end  of  Silurian  times.  These  (the  Caledonian  folds)  had  a 
general  trend  from  S.W.  to  N.E.,  and  were  accompanied  by 
violent  faulting  and  overthrusting,  together  with  intense 
dynamic  metamorphism.  From  this  time  dates  the  greater 
part  of  the  highly  complex  structure  of  the  Scottish  Highlands, 
as  well  as  the  present  disposition  of  the  older  rocks  of  the  Lake 
District,  north  and  central  Wales  and  a  large  part  of  Ireland. 
Earth-movements  on  so  large  a  scale  are  always  accompanied 
by  uplift  and  great  denudation,  especially  from  the  tops  of  the 
folds.  The  loose  material  thus  formed  is  accumulated  in  the 
intervening  hollows  and  gives  rise  to  continental  deposits, 
such  as  the  Old  Red  Sandstone.  Although,  as  above  stated, 
the  exact  nature  of  these  basins  of  deposition  is  not  known, 
they  have  for  convenience  been  called  lakes,  and  a  name  has 
been  assigned  to  each. 

The  Welsh  Lake.  The  Old  Red  Sandstone  rocks  are 
exposed  at  the  surface  over  a  large  area  in  the  border  counties 
and  in  South  Wales.  They  possess  a  total  thickness  of  severa1 
thousand  feet,  the  lower  part  consisting  of  red  and  variegated 
sandstones  and  marls  with  occasional  thin  bands  of  limestone, 
known  locally  as  "  cornstones."  The  upper  part  is  formed 


XT]  CARBONIFEROUS   SYSTEMS  221 

chiefly  of  red,  brown  and  yellow  sandstones  and  conglomerates, 
the  latter  being  largely  formed  of  pebbles  of  white  quartz. 
The  divisions  usually  adopted  are  as  follows : 

Upper  Division  ...     Yellow  and  red  sandstones  and  conglomerates, 

500  feet. 
Middle  Division  ...     Brown  sandstones    (Brownstone    series),    500 

to  1500  feet. 
Lower  Division  ...     Cornstone  series,  3000  to  4000  feet. 

The  only  fossils  are  fish,  such  as  Cephalaspis,  Pteraspis, 
Scaphaspis,  and  near  the  top  a  very  few  examples  of  Conularia, 
a  marine  shell,  show  an  occasional  and  limited  connexion  with 
the  sea. 

The  Old  Red  Sandstone  of  this  area  yields  soils  of  extra- 
ordinary fertility.  The  sandstones  and  marls  give  rise  to  red 
and  brown  soils  of  great  depth,  which  are  very  largely  under 
permanent  pasture,  affording  probably  the  richest  grass  land 
in  the  whole  of  the  British  Isles;  this  is  the  home  of  the  far- 
famed  Hereford  cattle.  It  is  generally  conceded  that  the 
Teme  valley  in  Herefordshire  contains  the  most  fertile  soils  of 
this  country,  and  both  here  and  in  Worcestershire  there  are 
extensive  hop-gardens,  comprising  about  one-third  of  the  total 
area  under  this  crop.  Throughout  the  lower  Severn  valley 
there  are  extensive  orchards  of  cider-apples.  The  Cornstones 
yield  soils  of  a  more  brashy  nature,  better  suited  for  corn 
growing;  hence  the  name.  On  some  of  the  high  ground 
occupied  by  the  uppermost  division  of  the  Old  Red  Sandstone, 
and  especially  on  the  conglomerates,  the  soils  are  of  poor 
quality,  being  very  stony.  The  great  elevation  is  also 
unfavourable  to  high -class  farming,  especially  in  the  Brecknock 
Beacons.  The  lower  Severn  valley  is  on  the  whole  a  district 
of  high  rents  and  high  farming,  contrasting  strongly  with  the 
areas  of  Lower  Palaeozoic  rocks  to  the  west.  There  are  however 
even  here  occasional  patches  of  poor  land  where  the  underlying 
rocks  consist  of  nearly  pure  quartz  sand,  without  the  admixture 
of  other  decomposed  minerals  that  contribute  so  much  to 
general  fertility. 

The  most  northerly  exposure  of  the  Old  Red  Sandstone  in 
this  area  is  a  few  miles  north  of  Bridgnorth,  in  Shropshire. 


222  THE  DEVONIAN  AND  [CH. 

Between  this  point  and  the  Scottish  border  the  formation  is 
only  represented  by  a  few  scattered  patches  of  conglomerate 
in  west  Yorkshire  and  Westmorland,  and  even  these  have  been 
referred  to  the  base  of  the  Carboniferous.  They  probably 
belong  to  the  Old  Red  Sandstone,  but  are  of  little  agricultural 
importance,  as  they  cover  only  a  very  small  area. 

Further  north  the  Old  Red  Sandstone  is  clearly  divisible 
into  two  series,  Upper  and  Lower,  there  being  usually  a  strongly- 
marked  unconformity  between  them ;  indeed  in  parts  of  Scotland 
horizontal  strata  of  the  Upper  series  can  be  seen  resting  on  an 
eroded  surface  formed  by  almost  vertical  beds  of  the  Lower 
series.  It  is  however  by  no  means  clear  whether  these  corre- 
spond in  time  to  the  upper  and  lower  divisions  of  the  Welsh 
borderland.  Some  writers  regard  the  "Upper  Old  Red  Sand- 
stone" of  Scotland  as  really  forming  the  base  of  the  Car- 
boniferous; this  question  is  not  yet  settled. 

In  the  Cheviot  area  both  series  are  represented;  the  older 
consists  partly  of  red  felspathic  sandstones  and  marls,  but  the 
greater  part  of  the  Cheviot  Hills  is  formed  by  igneous  rocks, 
including  both  lavas  and  ashes  and  a  large  boss  of  intrusive 
granite.  The  volcanic  rocks  are  2000  feet  thick,  and  include 
several  varieties  of  andesite  and  andesitic  ash.  The  Cheviot 
Hills  form  an  undulating  upland  region,  rising  to  the  height  of 
about  2600  feet,  and  chiefly  covered  with  short  fine  grass ;  this 
is  a  great  sheep-farming  district  and  is  the  original  home  of  the 
Cheviot  sheep,  which  have  now  spread  widely  over  the  Southern 
Uplands.  The  red  sandstones  and  marls  form  rich  agricultural 
land  in  Berwickshire  and  Roxburghshire,  especially  in  the 
Teviot  valley.  These  belong  mainly  to  the  Upper  Old  Red 
Sandstone,  forming  the  so-called  Tuedian  series ;  though  litho- 
logically  of  Old  Red  Sandstone  type,  they  may  be  wholly  of 
Carboniferous  age.  Their  outcrop  extends  continuously  into 
East  Lothian,  there  forming  some  of  the  most  fertile  and  most 
highly  rented  land  in  the  whole  of  the  British  Isles.  The  soil 
is  specially  suited  to  the  growth  of  potatoes  of  fine  quality. 
The  great  fertility  of  the  soil  is  no  doubt  largely  attributable 
to  the  presence  of  much  volcanic  material  in  the  rocks,  which  by 
its  gradual  decomposition  supplies  large  quantities  of  plant  food. 


xi]  CARBONIFEROUS   SYSTEMS  223 

Old  Red  Sandstone  rocks  form  the  surface  over  a  considerable 
area  in  the  central  valley  of  Scotland.  In  its  general  structure 
this  area  is  a  syncline,  the  older  rocks  cropping  out  along  the 
northern  and  southern  edges,  while  the  middle  part  is  occupied 
by  Carboniferous  strata.  Both  on  the  north  and  on  the  south 
the  valley  is  bounded  by  faults,  so  that  in  its  structure  it  is 
essentially  a  "  rift- valley."  The  sediments  of  Old  Red  Sandstone 
age  attain  a  great  thickness,  and  there  is  in  addition  a  large 
development  of  volcanic  rocks,  both  lavas  and  ashes.  The 
latter  attain  a  thickness  of  some  6000  feet  in  the  Ochil  and 
Sidlaw  Hills  on  the  north,  and  appear  also  in  the  Pentland 
Hills  south  of  Edinburgh.  The  volcanic  rocks  are  confined 
to  the  lower  division  of  the  system. 

The  Lower  Old  Red  Sandstone  of  Forfar,  which  probably 
attains  a  total  thickness  of  18,000  or  20,000  feet,  consists  of  a 
variable  series  of  sandstones,  conglomerates,  flagstones  and 
shales,  with  remains  of  fish  and  plants.  In  the  Ochil  Hills  the 
conglomerates  and  breccias  contain  many  blocks  of  volcanic 
rock,  indicating  the  existence  of  volcanoes  in  the  near  neigh- 
bourhood. The  prevailing  colour  of  the  sediments  is  red,  but 
some  of  the  flags  and  shales  are  grey.  The  volcanic  rocks  are 
chiefly  andesitic  in  character,  being  very  similar  to  those  of  the 
Cheviot  Hills. 

The  Upper  Old  Red  Sandstone,  lying  unconformably 
on  the  Lower  division,  consists  of  conglomerates  and  red 
sandstones  below,  with  occasional  beds  of  limestone,  the 
upper  part  being  yellow  sandstones  and  shales.  The  thickness 
is  about  2000  feet.  The  uppermost  division  contains  many 
fish-remains,  the  most  famous  locality  being  at  Dura  Den, 
near  Cupar. 

The  Old  Red  Sandstone  forms  a  broad  tract,  extending 
from  Helensburgh  and  Dumbarton  on  the  Clyde,  to  the  east 
coast  at  Stonehaven,  and  having  a  maximum  width  of  some 
25  miles.  This  includes  the  well-known  agricultural  districts  of 
Strathmore  and  the  Carse  of  Gowrie,  comprising  some  of  the 
most  highly  farmed  land  in  the  British  Isles.  'The  fertility  of 
the  soil  is  no  doubt  largely  due  to  the  presence  of  much  volcanic 
material,  just  as  in  the  case  of  the  Lothians.  In  many  parts 


224  THE  DEVONIAN  AND  [CH. 

however  the  actual  soil  is  formed  from  boulder-clay  and  other 
glacial  deposits. 

Scattered  patches  of  Old  Red  Sandstone  sediments  occur  in 
several  places  in  the  central  and  southern  Highlands,  showing 
that  the  strata  once  extended  further  than  they  do  now,  having 
been  largely  removed  by  denudation.  In  the  western  Highlands, 
in  the  neighbourhood  of  Oban  and  Glencoe,  there  is  also  a  con- 
siderable area  of  rocks  of  the  same  age,  largely  volcanic,  but 
with  some  remarkably  coarse  conglomerates.  None  of  these 
are  of  any  agricultural  importance. 

Along  both  shores  of  the  Moray  Firth,  in  Caithness  and  in 
the  Orkneys,  Old  Red  Sandstone  strata  cover  a  considerable  area. 
They  seem  to  belong  to  the  lower  division  of  the  series,  but  are 
of  a  type  somewhat  different  from  that  found  in  other  parts  of 
Scotland.  Red  sandstones  and  conglomerates  are  still  seen, 
especially  near  the  base,  but  the  main  mass  consists  of  shales 
and  flagstones,  the  whole  group  being  known  collectively  as  the 
Caithness  Flags.  The  total  thickness  is  about  16,000  feet. 
Fossil  fish  are  very  abundant.  The  upper  division  is  scantily 
represented  by  a  few  hundred  feet  of  yellow  and  red  sandstones 
in  Moray  and  Nairn,  with  another  small  patch  near  Dornoch. 
There  is  as  usual  a  marked  unconformity  at  the  base. 

The  soils  yielded  by  the  Old  Red  Sandstone  formation  in 
this  area  are  again  of  great  fertility  and  this  is  all  the  more 
remarkable  in  view  of  the  comparatively  high  latitude,  57°  to 
59°  N.  This  applies  with  almost  equal  force  to  all  the  Scottish 
areas  of  Old  Red  Sandstone,  all  those  on  the  eastern  side  of 
the  country  being  districts  of  great  fertility,  with  high  farming. 
This  state  of  things  is  due  to  a  combination  of  several  factors, 
the  chief  being  (a)  the  inherent  nature  of  the  rocks,  (6)  the 
comparatively  low  elevation,  and  (c)  the  climate.  The  presence 
of  a  thick  covering  of  drift  over  most  of  the  low  ground  has  also 
an  important  influence  in  modifying  the  character  of  the  soil, 
by  mixing  together  materials  of  varying  composition  and  thus 
supplying  all  the  elements  of  plant  food,  often  in  a  partially 
decomposed  and  readily  available  form. 

The  Old  Red  Sandstone  sediments  were  largely  formed 
by  denudation  of  crystalline  rocks  in  a  dry  climate,  so  that 


xi]  CARBONIFEROUS  SYSTEMS  225 

there  was  little  removal  of  soluble  matter  in  solution.  There 
was  also  in  many  places  a  considerable  admixture  of  con- 
temporaneous volcanic  material  and,  as  is  well  known,  the 
decomposition  of  lava  nearly  always  yields  a  rich  soil.  The 
rocks  are  also  of  such  a  nature  as  to  yield  naturally  a  deep, 
free-working  and  well-drained  soil,  easy  to  cultivate  and  easily 
penetrated  by  the  roots  of  plants.  Thus  all  the  natural  elements 
of  fertility  are  present.  But  these  alone  are  not  sufficient  to 
give  rise  to  good  land  unless  other  conditions,  especially  the 
climate,  are  favourable.  To  show  this  we  have  only  to  turn  to 
the  tracts  covered  by  the  Torridon  Sandstone  along  the  west 
coast  of  Sutherland,  Ross  and  Cromarty,  in  the  same  latitude 
as  the  shores  of  the  Moray  Firth.  This  formation  is  lithologically 
very  like  the  Old  Red  Sandstone  and  was  indeed  once  believed 
to  form  part  of  it,  though  this  is  now  known  to  be  erroneous. 
The  Torridon  Sandstone  forms  high,  bleak  mountains  and 
plateaux,  covered  with  heather  and  peat,  and  hardly  cultivated. 
The  climate,  though  not  really  very  cold,  is  wet  and  sunless,  and 
corn  can  scarcely  be  got  to  ripen  even  at  sea-level  in  this  district. 
The  only  corn  crop  cultivated  to  any  extent  is  oats. 

But  when  we  turn  to  the  Old  Red  Sandstone  tract  of  Caith- 
ness a  remarkable  contrast  is  seen.  The  land  is  well  suited  to 
the  growth  of  roots  and  barley,  and  distilling  is  therefore  an 
important  industry,  while  cattle  feeding  is  carried  on  extensively. 
The  lower  elevation  of  the  ground  certainly  has  a  considerable 
influence  here,  but  it  appears  that  the  most  important  factor 
is  climatic,  namely,  the  smaller  rainfall  with  correspondingly 
greater  amount  of  sunshine.  Extremes  of  climate  are  not  in 
themselves  inimical  to  the  growth  of  good  crops,  as  shown  by 
the  great  fertility  of  parts  of  the  prairie  region  of  America. 
The  long  cold  winter  is  counterbalanced  by  the  greater  heat 
and  sunshine  of  summer,  and  the  same  applies  in  a  lesser  degree 
to  the  east  coast  of  Scotland,  where  the  growing  season,  though 
short,  is  sunny,  and  the  summer  days  are  very  long.  Never- 
theless it  is  clear  that  the  character  of  the  soils  themselves  is 
of  the  greatest  importance  in  determining  their  fertility  and 
agricultural  value.  Good  climatic  conditions  alone  will  not 
make  all  soils  equally  fertile,  though  they  enhance  greatly  the 

R.  A.  G.  15 


226  THE  DEVONIAN  AND  [CH. 

value  of  a  naturally  good  soil,  such  as  is  yielded  by  the  Old  Red 
Sandstone  of  Scotland. 


II.     THE  CARBONIFEROUS  SYSTEM 

Where  the  Upper  Old  Red  Sandstone  is  present  it  is  com- 
monly overlain  conformably  by  the  Carboniferous;  indeed,  as 
before  stated,  the  actual  line  of  demarcation  between  the  two 
systems  is  in  many  places  quite  uncertain.  But  the  Carboni- 
ferous system  overlaps  the  Old  Red  Sandstone,  and  over  very 
large  areas  rests  unconformably  on  older  rocks,  either  Lower 
Palaeozoic  or  even  Precambrian.  The  rocks  of  the  Car- 
boniferous form  a  larger  proportion  of  the  surface  of  the  British 
Isles  than  any  other  formation,  and  in  addition  their  economic 
products  are  of  greater  commercial  importance  than  those  of 
any  other  system.  The  coal-fields  are  all  regions  of  concentrated 
industrial  activity  and  thickly  populated ;  hence  the  agricultural 
conditions  are  of  a  somewhat  special  character. 

The  rocks  of  the  Carboniferous  system  show  considerable 
variation,  both  vertically  and  horizontally.  The  most  constant 
features  are  the  preponderance  of  limestones  in  the  lower  part 
and  of  sandstones  and  shales  with  coal  in  the  upper  part, 
though  even  in  these  respects  there  are  local  variations  of  facies. 
For  example,  the  Carboniferous  rocks  of  Devonshire  belong  to 
a  special  facies,  the  Culm  type,  unknown  elsewhere  in  Britain, 
though  recurring  on  the  continent.  Again  in  Scotland  the 
coal-measure  facies  extends  into  the  lower  part  of  the  system. 
These  variations  of  facies  are  due  to  the  geographical  conditions 
prevailing  at  the  time  of  formation.  At  the  end  of  Old  Red 
Sandstone  times  the  British  area  was  subjected  to  a  general 
tilting  movement,  producing  fairly  deep  sea  in  the  south, 
gradually  shallowing,  with  local  irregularities,  towards  the 
north.  Beyond  the  present  southern  border  of  the  Highlands 
there  was  a  great  northern  continent,  the  source  of  much  of  the 
sedimentary  material.  As  time  went  on  this  difference  was 
gradually  obliterated  and  at  the  end  of  the  period  the  whole 
area  south  of  the  Highlands  consisted  of  low-lying  land  and 
shallow  water,  with  abundant  swamps  and  lagoons,  giving  rise 


xi]  CARBONIFEROUS   SYSTEMS  227 

to  the  conditions  necessary  for  coal-formation.  'The  highest 
Carboniferous  strata  give  indications  of  the  approach  of  a 
continental  phase,  which  culminated  in  the  succeeding  Permian 
and  Triassic  periods. 

The  Carboniferous  system  is  divided  into  two  main  series, 
the  Lower  and  Upper  Carboniferous;  these  in  most  localities 
differ  so  much  that  it  is  convenient  to  treat  each  separately. 
This  plan  greatly  facilitates  comparison  of  different  areas. 

A.    The  Lower  Carboniferous 

The  Lower  Carboniferous  strata  of  Devonshire  consist  of  a 
very  thin  series  of  peculiar  character,  chiefly  composed  of 
black  limestones  and  cherts,  the  whole  probably  formed  in  deep 
water.  Since  the  total  thickness  is  only  a  few  hundred  feet  and 
the  strata  are  highly  inclined,  the  outcrop  forms  only  a  very 
narrow  band  along  each  side  of  the  Devon  syncline,  and  the 
series  is  of  no  agricultural  importance.  The  chief  fossils  are 
Posidonomya  Becheri  and  Goniatites  (a  kind  of  primitive 
Ammonite). 

Lower  Carboniferous  rocks  outcrop  at  the  surface  over  large 
areas  in  Somerset,  Gloucestershire  and  South  Wales,  and  their 
development  is  very  complete;  in  fact  the  Bristol  district  must 
now  be  regarded  as  the  type  area  for  this  formation.  The 
typical  section  is  that  so  well  seen  in  the  Avon  gorge  between 
Bristol  and  the  sea.  The  Lower  Carboniferous  has  here  been 
divided  into  zones  characterized  by  corals  and  brachiopods; 
these  zones  are  of  wide  application  elsewhere  in  this  country. 
The  total  thickness  is  a  little  over  2000  feet  and  the  rocks  were 
originally  divided  into  the  Lower  Limestone  Shales  below  (350 
feet)  and  the  Carboniferous  Limestone  proper  above.  These 
divisions,  though  unscientific,  are  practically  useful,  since  they 
correspond  to  variations  of  lithological  character.  The  main 
mass  consists  of  highly  fossiliferous  marine  limestone  of  varying 
texture  and  colour,  with  occasional  shale  bands.  The  general 
colour  is  some  shade  of  grey  or  even  black  and  the  limestones 
are  very  massive,  consisting  largely  of  broken  shells  and  corals, 
with  Foraminifera.  The  corals,  Lithostrotion  and  Syringopora, 

15—2 


228  THE  DEVONIAN  AND  [CH. 

are  common,  with,  species  of  Productus,  Spirifer  and  allied 
genera  of  brachiopods.  The  highest  beds  are  shaly  and  sandy, 
passing  up  gradually  into  the  overlying  Millstone  Grit.  The 
Carboniferous  Limestone  also  forms  the  greater  part  of  the 
Mendip  Hills,  where  its  characteristic  features  are  particularly 
well  seen  at  Cheddar.  North  of  Bristol  the  series  becomes 
much  thinner;  round  the  Forest  of  Dean  coalfield  it  is  only 
about  600  feet  thick  and  near  Newport  it  appears  to  be  reduced 
to  350  feet,  but  thickens  again  westward.  All  over  this  area 
the  dips  are  as  a  rule  fairly  steep,  so  that  outcrops  are  narrow. 
In  the  southern  part  of  Pembrokeshire,  near  Tenby,  the  strata 
are  in  many  places  vertical. 

In  several  of  the  Midland  coal-fields  Lower  Carboniferous 
strata  are  completely  absent,  the  Coal-measures  resting  directly 
on  Silurian,  Cambrian  or  Precambrian  rocks.  This  area 
must  have  been  land  during  Lower  Carboniferous  times,  but 
it  is  uncertain  whether  it  was  an  island,  or  a  peninsula  extending 
eastwards  from  high  land  in  Wales.  Lower  Carboniferous 
rocks  again  appear  in  the  north  of  Shropshire,  and  reach  a 
considerable  development  in  Flint  and  Denbigh.  There  are 
also  a  few  scattered  patches  of  the  limestone  in  the  north  of 
Leicestershire.  But  in  Derbyshire  we  find  the  southern  limit 
of  the  largest  and  most  important  outcrop  of  Carboniferous  rocks 
in  the  British  Isles,  namely,  that  of  the  Pennine  range.  From 
a  little  north  of  Derby  Carboniferous  rocks  extend  uninterrupt- 
edly to  beyond  the  Scottish  border  and  the  width  of  the  outcrop 
is  considerable.  Between  Derbyshire  and  the  Border  the  rocks 
undergo  a  good  deal  of  lateral  variation ;  the  general  character 
of  the  change  can  perhaps  best  be  summed  up  by  saying  that 
the  further  north  we  go  the  lower  does  the  shallow  water  and 
terrestrial  facies  descend  in  the  series.  The  massive  marine 
limestones  of  the  south  are  gradually  replaced  by  shales  and 
grits  with  thin  impure  limestones,  indicating  a  shallowing  of 
the  water  towards  the  north.  In  the  north  of  Northumberland 
and  in  Berwick  the  total  amount  of  limestone  is  very  small.  In 
Derbyshire  the  base  of  the  system  is  not  seen  and  the  total 
thickness  of  the  Carboniferous  Limestone  is  unknown,  but  it 
is  noteworthy  that  though  the  visible  thickness  is  some  2500  or 


xi]  CARBONIFEROUS   SYSTEMS  229 

even  3000  feet,  the  whole  is  shown  by  its  fossils  to  belong  only 
to  the  uppermost  zones  of  the  Bristol  area.  In  west  Yorkshire, 
near  Settle  and  Ingleton,  the  base  of  the  Carboniferous  is  seen, 
resting  on  Silurian  and  older  rocks.  The  base  of  the  series  here 
occurs  at  a  horizon  roughly  equivalent  to  the  middle  of  the 
Bristol  succession,  whereas  in  Westmorland  most  of  the  series 
is  again  represented.  Hence  it  is  clear  that  the  Carboniferous 
Limestone  was  deposited  on  an  uneven  surface  and  that 
deposition  began  in  different  regions  at  widely  varying  times, 
as  the  ancient  land  was  denuded  away  or  submerged;  in  the 
Midlands  deposition  did  not  commence  at  all  till  Upper  Car- 
boniferous times. 

In  west  Yorkshire  the  Carboniferous  rocks  are  affected  by 
some  important  faults,  the  most  noteworthy  being  the  Craven 
faults,  near  Ingleton  and  Settle.  It  is  remarkable  that  the 
development  of  the  strata  shows  conspicuous  differences  on 
either  side  of  the  faults,  suggesting  that  even  at  that  time  some 
kind  of  natural  barrier  existed.  South  of  the  Craven  Faults, 
near  Clitheroe,  the  Lower  Carboniferous  series,  which  is  highly 
folded,  is  over  3200  feet  thick.  North  of  the  fault  at  Ingle- 
borough  the  maximum  thickness  of  the  same  strata  is  not  more 
than  1400  feet,  and  the  lithological  character  is  different. 
This  point  will  be  referred  to  again  in  connexion  with  the 
Millstone  Grit.  In  Westmorland  and  Cumberland  the  limestone 
rests  on  a  considerable  thickness  of  very  coarse  red  conglomerate, 
which  is  probably  of  Old  Red  Sandstone  age.  In  many  places, 
e.g.  in  the  Cross  Fell  escarpment,  the  base  of  the  undoubted 
Carboniferous  is  formed  by  a  conglomeratic  sandstone;  above 
this  comes  the  main  mass  of  limestone,  the  Melmerby  Scar 
limestone. 

Throughout  the  Pennine  range  the  uppermost  part  of  the 
Lower  Carboniferous  is  formed  by  a  series  of  shales  and  grits 
with  thin  limestones,  known  collectively  as  the  Yoredale  series. 
These  beds  show  the  oncoming  of  Upper  Carboniferous  con- 
ditions. 

When  the  Lower  Carboniferous  is  followed  into  Scotland  the 
lateral  variations  of  facies  are  found  to  be  very  strongly  marked. 
The  lowest  division  near  the  Border  consists  of  some  2000  feet 


230 


THE  DEVONIAN  AND 


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xi]  CARBONIFEROUS   SYSTEMS  231 

of  red  sandstone,  named  the  Tuedian  series,  from  the  river 
Tweed,  while  the  calcareous  facies  has  completely  disappeared, 
being  replaced  by  rocks  like  the  Yoredales  of  Yorkshire.  In  the 
central  valley  of  Scotland  the  more  important  of  the  coal  seams 
are  found  in  the  upper  part  of  the  Lower  Carboniferous,  the 
Edge  Coal  group,  while  below  this  come  the  Cement-stone  and 
Oil-shale  groups,  passing  down  conformably  into  the  Upper 
Old  Red  Sandstone. 

The  succession  and  equivalence  of  the  Lower  Carboniferous 
rocks  of  the  Pennine  area  and  Scotland  can  be  most  clearly 
shown  in  a  tabular  form,  as  on  the  opposite  page. 

From  the  above  descriptions  it  is  clear  that  the  rocks  of  this 
series  show  wide  variations  of  character,  and  will  therefore 
yield  very  varying  soils.  This  subject  can  be  most  profitably 
discussed  after  the  Millstone  Grit  has  been  described,  since  the 
two  divisions  are  intimately  connected;  this  course  will  avoid 
some  repetition,  otherwise  inevitable. 

B.    The  Millstone  Grit 

Almost  everywhere  the  lowest  member  of  the  Upper 
Carboniferous  series  is  made  up  to  a  greater  or  less  extent  of 
sandstones  or  grits,  of  coarse  texture  and  sometimes  con- 
glomeratic in  their  nature.  Intercalated  with  beds  of  this 
type  are  shales  and  occasional  limestones.  The  general  name 
of  the  series  is  due  to  the  fact  that  some  of  the  grits  were  found 
specially  suitable  for  use  as  millstones,  owing  to  their  very 
rough  and  gritty  texture.  The  Millstone  Grit  type  of  deposit 
is  strictly  a  facies  rather  than  a  definite  time-division,  and  it 
is  quite  clear  that  both  the  top  and  the  bottom  of  the  division 
as  at  present  defined  are  at  different  horizons  in  different 
localities.  Hence  the  demarcation  of  the  Lower  and  Upper 
Carboniferous  presents  some  difficulties.  For  example,  the 
Millstone  Grit  of  Bristol  is  certainly  represented  in  part  by  the 
uppermost  zone  of  the  Carboniferous  Limestone  of  Flint  and 
Denbigh.  However  for  our  present  purpose  the  lithological 
change  is  of  the  most  importance,  and  the  term  Millstone  Grit 
will  here  be  employed  in  a  somewhat  old-fashioned  sense. 


232  THE   DEVONIAN  AND  [CH. 

Near  Bristol  and  in  South  Wales  the  strata  between  the  top 
of  the  limestone  and  the  base  of  the  Coal-measures  consist  of 
about  450  feet  of  yellow  sandstone,  usually  of  rather  fine 
texture.  Over  most  of  the  Midlands  this  division  is  absent, 
but  in  Denbigh,  Cheshire  and  Derbyshire  it  sets  in  again  and 
thickens  rapidly  towards  the  north,  reaching  a  total  thickness 
of  perhaps  4000  feet  in  east  Lancashire.  In  this  area  there  are 
now  recognized  two  series,  the  Pendleside  group  below,  and  the 
Millstone  Grit  above.  The  Pendleside  group  consists  chiefly  of 
shales  and  black  limestones,  and  was  formerly  regarded  as 
equivalent  to  the  Yoredale  beds  of  Yorkshire  and  therefore  of 
Lower  Carboniferous  age ;  it  is  now  recognized  from  its  fossils 
to  belong  to  the  Upper  Carboniferous.  The  Millstone  Grit 
proper  consists  of  an  alternation  of  massive  grits  and  black 
shales.  The  grits  often  form  conspicuous  escarpments  and 
table-lands,  such  as  Kinder  Scout  and  Blackstone  Edge.  North 
of  Settle  the  Pendleside  group  cannot  be  identified,  and  the 
Millstone  Grit  dwindles  down  to  600  feet  in  Yoredale,  though 
it  is  said  to  thicken  again  in  Durham  and  Northumberland, 
where  it  contains  several  workable  coal-seams.  In  Scotland 
the  Millstone  Grit  is  represented  by  the  "Moorstone  Rock,"  or 
Rosslyn  sandstone,  from  350  to  600  feet  thick,  near  Edinburgh, 
but  thinning  rapidly  to  the  west. 

From  the  foregoing  brief  account  it  is  clear  that  the  Millstone 
Grit  is  a  variable  formation,  including  rocks  of  many  different 
types,  likely  to  yield  soils  of  very  varying  character.  It 
appears  to  be  in  the  main  a  shallow-water  marine  facies,  fore- 
shadowing the  oncoming  of  the  continental  conditions  of  the 
Coal-measures. 

The  calcareous  facies  of  the  Lower  Carboniferous  is  one  of 
the  most  striking  rock-types  to  be  found  in  this  country. 
Where  present,  in  Great  Britain,  it  commonly  forms  regions  of 
considerable  elevation,  such  as  the  Mendips  and  the  hills  of 
Derbyshire  and  west  Yorkshire.  Hence  it  was  formerly  known 
as  the  Mountain  Limestone,  a  useful  name  now  generally 
abandoned.  It  is  only  in  Ireland  that  this  limestone  occurs  to 
a  large  extent  in  low  ground,  forming  the  substratum  of  a  great 
proportion  of  the  boggy  plain  in  the  centre  of  that  country. 


xi]  CARBONIFEROUS   SYSTEMS  233 

The  Carboniferous  Limestone  shows  very  perfectly  the  peculiar 
kind  of  denudation  characteristic  of  calcareous  rocks,  now 
generally  designated  as  the  Karst  type,  from  a  district  on  the 
eastern  side  of  the  Adriatic  (see  p.  71).  Where  Carboniferous 
Limestone  forms  a  level  area,  free  from  drift,  it  usually  bears  no 
soil,  the  surface  consisting  of  bare  rock,  with  many  and  deep 
open  fissures  called  "grikes"  in  Yorkshire;  in  these  alone  does 
any  vegetation  manage  to  exist.  This  state  of  affairs  is  well 
seen  on  the  "clints"  of  Ingleborough  and  near  Grange  in 
Westmorland.  Such  bare  rock  surfaces  are  common  in  all 
Carboniferous  Limestone  districts;  other  conspicuous  features 
are  steep  escarpments,  dry  valleys,  rock-pillars,  caves,  swallow- 
holes  and  underground  rivers.  All  of  these  are  well  seen  in 
Derbyshire  and  west  Yorkshire.  Where  some  soil  does  exist 
on  the  limestone  it  is  generally  covered  with  short,  sweet  turf, 
heather  and  bracken  being  almost  unknown  even  at  high 
elevations.  In  parts  of  Yorkshire  the  limestone  is  masked  by 
drift;  under  these  circumstances  the  character  of  the  soil  and 
vegetation  is  modified  and  peat  may  be  formed.  Even  then 
however  the  underground  drainage  has  an  important  influence 
on  the  superficial  formations,  and  the  surface  often  shows  many 
funnel-shaped  hollows,  due  to  the  existence  of  swallow-holes 
below. 

The  Lower  Carboniferous  rocks  and  the  Millstone  Grit  of 
the  Pennine  Chain  form  a  great  mass  of  high  ground,  dissected 
by  deep  valleys  and  often  rising  into  mountains,  as  in  Derby- 
shire and  west  Yorkshire.  On  the  borders  of  Westmorland, 
Durham,  Cumberland  and  Northumberland  is  a  great  plateau 
over  2000  feet  in  height,  the  highest  point,  Cross  Fell,  nearly 
attaining  3000  feet.  In  the  southern  part  of  the  range  lime- 
stones are  dominant,  showing  the  characteristic  features  before 
described,  but  further  north  the  strata  consist  chiefly  of  an 
alternation  of  massive  grits  and  shales,  with  only  subordinate 
calcareous  bands.  The  grits  and  shales  of  the  higher  regions 
are  very  largely  open  moorland,  often  very  peaty,  and  covered 
with  heather.  To  the  south-west  of  the  Craven  fault  is  a  large 
extent  of  comparatively  low  ground,  composed  of  Millstone 
Grit,  Pendleside  and  Yoredale  beds.  The  latter  division  is  also 


234  THE   DEVONIAN   AND  [CH. 

prominent  on  the  eastern  side  of  the  watershed,  and  in 
Wensleydale  it  forms  an  area  of  fertile  land,  largely  devoted 
to  dairying  (cheese-making).  The  soils  derived  from  the  grit 
rocks  are  often  stony  or  light  and  sandy,  but  the  shales  are 
somewhat  heavier  and  of  better  quality,  though  often  in  need 
of  drainage. 

C.    The  Coal-measures 

From  the  economic  standpoint  this  division  of  the  Upper 
Carboniferous  is  the  most  important  formation  in  Great 
Britain.  It  forms  the  surface  of  several  very  considerable 
areas,  and  its  underground  extension  is  also  known  to  be  great 
beneath  newer  rocks.  With  the  buried  coal-fields  we  are  not 
here  concerned,  except  in  a  very  general  way.  The  Coal- 
measures  do  not  as  a  whole  form  very  good  agricultural  land, 
but  owing  to  the  density  of  population  those  parts  not  actually 
built  over  are  almost  entirely  under  cultivation  of  one  sort  or 
another. 

The  Coal-measures  may  be  regarded  as  a  continuation  and 
exaggeration  of  the  shallow- water  and  continental  facies  of  the 
Millstone  Grit,  and  of  the  Lower  Carboniferous  of  the  northern 
areas.  In  fact,  as  already  pointed  out,  Coal-measure  conditions 
set  in  during  the  Lower  Carboniferous  period  in  Scotland. 
The  Coal-measures  consist  of  a  very  variable  series  of  sand- 
stones, clays  and  shales,  with  seams  of  coal  and  occasional 
bands  of  marine  limestone,  indicating  temporary  incursions  of 
the  sea.  In  parts  of  the  Midlands,  in  Staffordshire  and  in 
Warwickshire,  the  uppermost  beds  are  of  a  very  different  type, 
consisting  of  red  sandstones,  marls  and  conglomerates,  with 
little  or  no  coal,  indicating  the  approach  of  a  terrestrial  phase. 

The  present  disposition  of  the  coal-fields  of  England  and 
Wales  is  the  result  of  earth-movements  of  the  Armorican 
series,  that  took  place  after  the  Carboniferous  period,  but  before 
the  Permian.  There  were  then  formed  two  sets  of  folds,  striking 
E.-W.  and  N.-S.  respectively,  the  former  being  dominant 
in  the  south,  the  latter  in  the  north.  Erom  the  tops  of  the 
anticlines  thus  formed  the  Upper  Carboniferous  rocks  were 


xi]  CARBONIFEROUS   SYSTEMS  235 

removed  by  denudation,  the  Coal-measures  being  preserved  in 
the  intervening  basins. 

The  visible  coal-fields  of  Great  Britain  may  be  conveniently 
divided  into  four  groups,  as  follows: 

1.  South  Wales,  Forest  of  Dean  and  Bristol. 

2.  The  Midlands. 

3.  The  Pennines. 

4.  The  central  valley  of  Scotland. 

There  is  a  large  area  of  Upper  Carboniferous  rocks  forming 
the  centre  of  the  Devon  syncline,  but  these  belong  to  the  Culm 
facies  and  do  not  contain  workable  coal-seams. 

The  Coal-measures  of  north  Somerset  do  not  form  a  large 
area  at  the  surface,  the  coal  being  largely  worked  below  a  cover 
of  Secondary  rocks,  but  the  South  Wales  coal-field  is  one  of  the 
largest  in  the  country.  It  formed  originally  a  high-lying 
plateau,  now  deeply  dissected  by  numerous  north  and  south 
valleys,  such  as  Taff  Vale  and  the  Rhondda  valley.  The  general 
structure  is  an  oval  basin,  with  its  long  axis  running  east  and 
west,  but  there  are  several  subsidiary  folds,  throwing  the  rocks 
into  undulations.  In  Pembrokeshire  the  structure  is  more 
complex  and  folding  and  faulting  more  strongly  marked. 

Three  divisions  are  generally  recognized  in  the  Coal -measures' 
of  this  area: 

Upper  Coal-measures        ...       3000  feet. 

Pennant  Grit          3000      „ 

Lower  Coal-measures        ...       1600      „ 

It  is  to  be  noted  that  the  "Upper"  Coal-measures  of  South 
Wales  are  equivalent  to  the  upper  part  of  the  Middle  Coal- 
measures  of  the  Midland  coal-fields.  The  Lower  and  Upper 
measures  both  contain  abundant  seams  of  coal,  interstratified 
with  shales  and  some  sandstones,  while  the  Pennant  series  is 
mainly  composed  of  felspathic  grits,  with  little  or  no  coal, 
except  in  the-  west,  near  Swansea. 

The  Midland  group  of  coal-fields  includes  several  scattered 
patches  of  no  great  individual  size,  but  collectively  of  much 
industrial  importance.  They  are  mainly  situated  in  Warwick- 
shire, Leicestershire,  south  Staffordshire  and  Shropshire.  For 
the  most  part  they  rest  directly  on  older  rocks,  the  Lower 


236  THE   DEVONIAN   AND  [CH. 

Carboniferous  being  absent.  In  all  these  areas  the  "pro- 
ductive" Coal-measures  consist  of  grey  or  white  sandstones, 
with  good  seams  of  coal,  one  seam  in  Warwickshire  being  25  feet 
thick.  The  upper  Coal-measures,  where  present,  are  usually  red 
in  colour,  without  coal,  and  a  large  part  of  them  have  been 
mapped  as  Permian  by  the  Geological  Survey. 

The  coal-fields  of  the  Pennine  group  include  the  following : 

North  Staffordshire, 

Lancashire, 

Yorkshire,  Derbyshire  and  Nottinghamshire, 

Durham  and  Northumberland, 

Cumberland. 

The  Denbigh  and  Flint  coal-field  is  the  western  limb  of  a 
syncline  whose  eastern  limb  forms  the  north  Staffordshire 
coal-field.  The  beds  are  no  doubt  continuous  below  the  plain 
of  Cheshire,  though  too  deep  in  the  centre  to  be  workable. 

The  Coal-measures  of  north  Staffordshire  and  Lancashire 
show  a  general  succession  as  follows : 

Upper  Coal-measures        Red  and  purple  sandstones      2000  feet. 

and  marls 
Middle  Coal-measures       Grey  shale  and   sandstones       3500      „ 

with  coal  and  ironstone 
Lower  Coal-measures        Grey  flags  and  black  shales       1800      „ 

with  thin  coals 

Workable  coal-seams  are  almost  confined  to  the  middle 
division;  in  south  Lancashire  the  total  thickness  of  coal  is 
from  70  to  80  feet,  and  workable  beds  of  ironstone  are  abundant. 
The  red  measures  are  well  developed  in  the  Potteries  coal-field 
in  north  Staffordshire,  but  they  are  not  well  seen  in  Lancashire. 
They  contain  no  coal. 

The  Coal-measures  form  the  surface  rock  over  a  large  area  in 
Nottinghamshire,  Derbyshire  and  Yorkshire,  and  they  are  known 
to  extend  underground  far  to  the  east.  The  lower  division  both 
here  and  in  Lancashire  is  often  called  the  Gannister  series,  this 
being  a  miners'  term  for  a  hard  white  siliceous  sandstone  that 
often  occurs  below  the  coal-seams.  The  middle  Coal-measures 
have  a  very  great  thickness  and  are  closely  similar  to  those  of 
Lancashire,  many  of  the  seams  being  the  same.  The  red 


xi]  CARBONIFEROUS  SYSTEMS  237 

measures  are  only  known  in  borings  in  Nottinghamshire  and 
near  Rotherham. 

The  Gannister  beds  become  very  thin  in  Durham  and 
Northumberland,  probably  not  more  than  150  feet,  but  the 
Middle  Coal-measures  are  some  2000  feet  thick,  with  about 
twenty  seams  of  coal.  It  is  not  known  whether  any  Upper  Coal- 
measures  are  present  in  this  area.  The  Cumberland  coal-field 
consists  only  of  a  small  patch  near  Whitehaven,  the  relic  of  a 
once  much  larger  area  that  was  probably  destroyed  during 
Tertiary  times  by  the  uplift  of  the  Lake  District  dome. 

The  Upper  Carboniferous  series  is  not  well  developed  in 
Scotland,  where  most  of  the  productive  coal-seams  are  in  the 
lower  division  of  the  system.  In  the  Clyde  basin  the  Lower 
Coal-measures  contain  ten  workable  seams,  with  several  import- 
ant bands  of  ironstone.  The  Upper  Coal-measures  of  Scotland 
are  red  and  unproductive,  and  their  relation  to  the  strata  of  the 
English  coal-fields  is  still  uncertain ;  they  rest  unconformably 
on  the  lower  division. 

In  Ireland  Coal-measures  are  found  in  Kilkenny  and  Queen's 
County,  in  Leinster,  where  there  are  a  few  good  seams  of  coal, 
and  also  in  Clare,  Limerick,  Kerry  and  Cork,  forming  the 
Munster  coal-field,  where  the  seams  are  thin  and  variable.  The 
rocks  consist  of  sandstones,  flags  and  shales  and  have  a  maximum 
thickness  of  about  3500  feet  in  Munster  and  1700  feet  in  Leinster. 
Commercially  they  are  unimportant. 

The  fossils  of  the  Coal-measures  differ  greatly  from  those  of 
the  Carboniferous  Limestone,  belonging  chiefly  to  those  types 
characteristic  of  estuarine,  lagoon  and  fresh- water  conditions. 
By  far  the  most  important  are  remains  of  plants,  together 
with  fresh-water  mollusca,  fish,  amphibia,  Crustacea  and  even 
insects.  Here  and  there  are  found  bands  of  limestone  with  a 
rather  scanty  marine  fauna.  These  marine  bands  however  are 
very  useful  for  purposes  of  correlation  and  serve  as  indices  in 
locating  the  horizons  of  the  different  coal-seams.  The  flora  of 
the  Coal  Measures  includes  gigantic  lycopods  (club  mosses) 
such  as  Lepidodendron  and  Sigillaria ;  the  roots  of  the  latter  are 
often  called  Stigmaria.  Calamites  is  a  large  species  of  Horse- 
tail (Equisetaceae).  True  conifers  are  also  known,  but  some  of 


238  THE   DEVONIAN  AND  [CH. 

the  most  abundant  forms,  formerly  classed  with  the  ferns,  are 
now  known  to  be  intermediate  between  the  ferns  and  the 
cycads ;  Alethopteris,  Neuropteris,  Pecopteris  and  others.  Among 
the  fresh- water  mollusca  the  most  important  are  Carbonicola, 
Anthracomya  and  Naiadites.  The  marine  fauna  includes  many 
forms  common  also  in  the  Millstone  Grit  and  Pendleside  series, 
some  being  found  in  the  Lower  Carboniferous.  Some  of  the 
more  important  are  Goniatites,  Bellerophon,  Posidonomya 
Becheri,  Aviculopecten  papyraceus,  Productus  semireticulatus, 
and  Spirifer  Urei. 

Since  the  strata  composing  the  Coal-measures  show  much 
variation  of  lithological  character  the  soils  naturally  also  vary 
very  widely.  The  chief  rock-types  are  sandstone,  shale  and 
clay,  often  occurring  in  comparatively  thin  beds  in  rapid  alter- 
nation. In  general  terms  it  may  be  said  that  the  sandstones 
yield  light  soils,  the  shales  and  clays  heavy,  cold  soils,  but  both 
types  agree  in  being  on  the  whole  of  poor  quality,  of  low  natural 
fertility.  Furthermore,  over  large  areas  the  growth  of  crops  is 
seriously  hampered  by  the  smoke  from  numerous  industrial 
centres  and  factories,  not  only  in  towns,  but  scattered  about 
the  country  districts.  It  is  surprising  over  what  wide  areas  this 
effect  can  be  traced.  Again  in  many  places  much  land  is  spoilt 
by  the  flow  of  surface  water  contaminated  with  poisonous 
substances  from  pit-banks,  slag-heaps  and  many  other  sources. 
Even  if  not  directly  poisonous  to  crops,  these  waters,  which  are 
often  strongly  acid,  help  to  dissolve  out  the  soluble  plant  food 
in  the  soil  and  especially  to  remove  the  lime,  which  as  a  rule 
is  naturally  deficient.  Nevertheless,  in  spite  of  many  draw- 
backs, owing  to  special  geographical  and  economic  causes,  much 
profitable  farming  is  found  on  the  Coal-measures.  It  is  however 
of  a  highly  specialized  type.  As  an  example  we  may  take  the 
West  Riding  of  Yorkshire,  a  typical  industrial  area  supporting 
a  vast  population.  In  such  an  area,  as  is  natural,  the  boundary 
between  farming  and  market  gardening  is  very  difficult  to 
draw,  and  certain  crops,  such  as  rhubarb,  usually  only  found  in 
gardens,  are  here  cultivated  as  field  crops.  Another  very 
special  industry  is  the  growing  of  green  crops  to  be  mown  as 
fodder  for  pit  ponies.  As  before  stated,  it  is  here  possible  to 


xij  CARBONIFEROUS   SYSTEMS  239 

recognize  two  distinct  types  of  soil,  heavy  and  light,  which  are 
often  sharply  marked  off  over  small  areas.  Arable  land 
predominates,  only  the  very  heavy  soils  being  under  permanent 
pasture  and  meadow.  In  many  parts,  especially  near  the  large 
towns,  practically  the  whole  produce  of  the  land  is  sold  off  and 
great  quantities  of  town  manure  are  purchased  to  maintain  a 
good  standard  of  imparted  fertility.  Potatoes  receive  special 
attention  and  are  grown  with  corn,  mainly  wheat,  and  clover, 
in  a  five-course  rotation.  In  districts  where  there  is  sufficient 
grass  the  production  of  milk  for  sale  is  of  considerable  import- 
ance, especially  near  towns.  In  the  more  remote  districts, 
away  from  railways,  there  is  a  more  ordinary  type  of  farming, 
but  the  district  is  not  good  for  sheep,  since  the  smoke  is 
injurious  to  them;  in  the  more  populous  districts  the  vast 
number  of  dogs  kept  by  the  inhabitants  render  sheep-farming 
a  somewhat  anxious  business. 

This  description  of  one  special  district  may  be  applied  with 
little  variation  to  most  of  the  British  coal-fields,  where  the 
industrial  conditions  rather  than  the  actual  character  of  the 
soil  usually  determine  the  type  of  farming.  There  are  however 
some  notable  variations.  The  Pennant  Grit  of  South  Wales, 
which  contains  no  workable  coal,  forms  an  elevated  moorland 
tract,  and  in  Gloucestershire  there  is  a  large  area  of  woodland 
(the  Forest  of  Dean).  The  uppermost  red  Coal-measures  of 
the  Midlands  form  soils  very  like  those  of  the  overlying  Trias. 
In  Devonshire  there  is  a  great  area  of  sandstones  and  shales 
without  coal  (the  Culm  measures)  that  form  poor  wet  clays,  or 
stony  sandy  soils.  There  is  a  good  deal  of  woodland  on  the  sides 
of  the  steep  valleys. 


CHAPTER  XII 

THE   PERMIAN  AND  TRIASSIC  SYSTEMS  ("NEW  RED 
SANDSTONE") 

The  highest  beds  of  the  Carboniferous  of  the  Midland  coal- 
fields show  a  prevailing  red  colour,  indicating  the  oncoming  of 
a  phase  of  continental  conditions.  This  was  the  result  of  the 
great  earth-movements  of  the  Armorican  series,  which,  as 
already  mentioned,  were  responsible  for  the  present  arrangement 
of  the  Coal-measures  in  isolated  basins.  For  a  time  denudation 
was  dominant  over  deposition,  removing  the  Carboniferous 
strata  from  the  anticlines  and  depositing  the  material  in  the 
intervening  synclines;  consequently  Permian  strata  occur  in 
isolated  patches  of  which  only  two  now  occupy  any  considerable 
area.  During  the  Triassic  period  the  arid  continental  type  of 
deposition  became  much  more  widespread  in  Britain,  and 
Triassic  strata  form  a  very  considerable  proportion  of  the 
surface,  especially  in  the  northern  and  western  Midlands. 

The  unconformity  between  the  Carboniferous  and  Permian 
strata  is  one  of  the  most  conspicuous  in  the  British  Isles  and  is 
seen  with  special  clearness  in  Nottinghamshire,  Yorkshire  and 
Durham.  This  unconformity  probably  represents  a  considerable 
lapse  of  time,  since  the  true  upper  Coal-measures  (Stephanian) 
of  France  and  Germany  appear  to  be  unrepresented  in  Britain. 
In  India  and  in  the  southern  hemisphere  there  is  no  break  in 
the  series,  as  for  example  in  the  Karroo  system  of  South  Africa 
and  the  Gondwana  system  of  India,  which  range  from  Car- 
boniferous to  Trias. 

In  conformity  with  their  origin  during  a  period  of  strongly 
marked  continental  conditions  the  strata  of  the  Permian  and 
Triassic  systems  possess  characters  strongly  resembling  those 


CH.XII]    THE  PERMIAN  AND  TRIASSIC  SYSTEMS      241 

of  the  deposits  now  being  laid  down  in  arid  regions;  in  fact 
throughout  both  systems  there  is  abundant  evidence  of  the 
prevalence  of  desert  conditions  in  this  country.  Marine  strata, 
where  developed  at  all,  are  peculiar,  with  a  scanty  fauna ;  beds 
of  rock-salt,  gypsum  and  dolomitic  limestone  occur  on  a  large 
scale,  and  even  the  sandstones  and  marls  show  undoubted 
evidence  of  wind-action;  in  places  the  pitted,  scored  and 
polished  surfaces  of  the  older  underlying  rocks  have  been 
preserved  to  this  day.  In  their  general  characters  the  Permian 
and  Trias  are  very  similar,  and  it  has  been  proposed  to  class 
them  all  together,  as  the  New  Red  Sandstone  system.  Litho- 
logically  and  stratigraphically  there  is  abundant  justification 
for  this,  but  unfortunately  in  their  marine  equivalents  in  other 
parts  of  the  world,  one  of  the  most  important  of  all  palaeonto- 
logical  breaks  occurs  in  the  middle  of  the  series ;  the  Palaeozoic 
fauna,  characterized  by  trilobites,  here  gives  way  to  the  Mesozoic 
ammonite  fauna.  Hence  although  there  is  little  or  no  strati- 
graphical  break,  the  dividing  line  between  the  Palaeozoic  and 
Mesozoic  epochs  has  been  drawn  at  this  horizon.  As  will 
appear  later,  the  demarcation  of  Permian  and  Trias  presents 
some  difficulty  in  Britain  and  if  this  country  alone  were  to  be 
considered  there  would  be  no  hesitation  in  the  adoption  of  the 
New  Red  Sandstone  system. 

I.     THE  PERMIAN  SYSTEM 

The  Permian  rocks  of  England  form  two  principal  areas, 
one  on  either  side  of  the  Pennine  chain,  with  some  smaller 
patches  in  the  Midlands  and  in  Devonshire;  as  before  men- 
tioned, most  of  the  red  rocks  formerly  mapped  as  Permian  in 
the  Midlands  are  now  known  to  be  Upper  Carboniferous.  The 
largest  area  of  Permian  rocks  forms  a  narrow  strip  running  in 
a  nearly  straight  line  from  near  Nottingham  to  the  mouth  of 
the  Tyne.  The  two  lower  divisions,  the  Yellow  sands  and  the 
Marl-slate,  are  quite  unimportant,  but  the  higher  member,  the 
Magnesian  Limestone  series,  has  a  maximum  thickness  of  about 
800  feet  in  Durham,  where  it  consists  chiefly  of  dolomite  rock, 
often  with  a  brecciated  or  concretionary  structure.  Towards 

B.A.  G.  16 


242  THE   PERMIAN   AND  [CH. 

the  south  it  becomes  thinner  and  beds  of  red  sandstone  and  marl 
come  in ;  in  Nottinghamshire  about  half  of  the  series  is  marly 
and  sandy,  and  the  visible  thickness  is  small,  owing  to  overlap 
of  the  Trias.  Fossils  are  scarce  in  the  Magnesian  Limestone,  as 
a  result  of  the  conversion  of  the  original  marine  limestone  into 
dolomite-rock.  The  most  common  are  Productus  horridus, 
Schizodus  obscurus,  Fenestella  retiformis  and  fish  (Palaeoniscus 
and  others). 

The  Permian  rocks  of  the  Eden  valley  in  Westmorland  and 
Cumberland  are  very  different  in  thickness  and  lithological 
character,  although  the  divisions  can  be  correlated,  as  shown 
in  the  following  table: 

Western  basin  Eastern  basin 

Magnesian  Limestone  0-30  ft.  Magnesian  Limestone    500-800  ft. 

Hilton  Shales           ...  150   „  Marl  Slate 3-  50   „ 

Penrith  Sandstone  ...  1500   „  Yellow  Sands          ...         0-100   „ 

The  Penrith  Sandstone  affords  a  fine  example  of  a  true 
desert  deposit.  The  maximum  thickness  near  Appleby  is 
about  1500  feet,  consisting  of  bright  red  false-bedded  sandstone, 
with  well-rounded  grains  and  other  indications  of  wind-trans- 
port. This  affords  an  excellent  building-stone.  Near  the  base 
and  summit  are  beds  of  breccia,  known  locally  as  "brockrams." 
These  consist  of  angular  fragments  of  Carboniferous  Limestone 
embedded  in  a  red  marly  or  fine  sandy  matrix.  The  fragments 
appear  to  have  been  derived  from  the  Cross  Fell  fault-scarp, 
which  must  have  been  in  process  of  formation  during  Permian 
times.  The  brockrams  are  thickest  in  the  south,  near  Kirkby 
Stephen  and  disappear  about  Penrith.  The  Hilton  Shales 
contain  plant  remains  like  those  of  the  Marl-slate.  The  so- 
called  Magnesian  Limestone,  really  a  sandstone  or  flagstone 
with  a  dolomite  cement,  is  quite  insignificant. 

There  are  very  small  patches  of  Permian  sandstones  at 
Ingleton,  Clitheroe  and  in  the  neighbourhood  of  Manchester, 
but  not  much  is  known  about  them.  In  the  Midland  counties 
the  only  undoubted  Permian  rocks  are  found  in  Shropshire 
(Enville  marls)  and  on  the  eastern  flanks  of  the  Malvern  Hills. 
These  are  not  thick  and  cover  only  a  very  small  area. 


xn]  TRIASSIC   SYSTEMS  243 

The  lower  part  of  the  great  series  of  red  sandstones  and 
conglomerates  so  conspicuous  in  Devonshire  is  considered  to  be 
of  Permian  age,  but  the  line  of  demarcation  from  the  Trias  is 
somewhat  uncertain.  These  rocks  are  splendidly  exposed  in 
the  cliffs  about  Dawlish  and  Teignmouth,  and  again  just 
beyond  Torquay.  They  consist  partly  of  bright  brick-red 
sandstones  and  partly  of  conglomerates  or  breccias,  composed 
of  large  and  small  fragments  of  older  rocks  of  many  kinds, 
embedded  in  a  red  sandy  matrix.  Near  Exeter  there  are  some 
igneous  rocks,  quartz-porphyry  and  basalt,  with  coarse  breccias. 
The  total  thickness  commonly  assigned  to  the  Permian  system 
in  Devonshire  is  about  2000  feet. 

Since  the  subdivisions  of  the  Permian  rocks  show  so  much 
variation  of  character  they  naturally  yield  soils  of  very  different 
quality.  The  Penrith  Sandstone,  when  not  covered  by  drift, 
yields  a  very  light  soil,  often  woodland  or  heathy  ground,  but 
it  is  generally  modified  by  a  cover  of  boulder-clay.  The  soils 
of  the  eastern  outcrop  also  vary  a  good  deal,  especially  in 
Nottinghamshire,  where  beds  of  sandstone  and  marl  partly 
replace  the  Magnesian  Limestone.  The  Magnesian  Limestone 
typically  yields  a  brown  loamy  soil  but  is  often  covered  by  a 
stiff  clay ;  this  is  possibly  due  to  relics  of  shales  or  marls 
which  formerly  covered  it,  but  have  been  mostly  removed  by 
denudation.  The  marls,  when  unmixed,  form  a  cold  tenacious 
soil,  but  when,  as  often  happens,  they  are  mixed  with  sands, 
the  result  is  a  deep  fertile  loam  yielding  good  land.  The  lower 
mottled  sandstones  form  a  very  light  sandy  soil.  When  followed 
northwards  into  Yorkshire  the  marls  and  sands  gradually 
disappear,  and  the  escarpment  of  the  Magnesian  Limestone 
forms  a  conspicuous  plateau,  declining  gently  to  the  east. 
This  is  very  well  seen  about  Ferrybridge.  This  area  carries 
rather  light  soils,  often  thin  and  of  somewhat  poor  quality; 
the  most  important  crops  are  barley  and  turnips.  Where  the 
soil  is  rather  more  loamy,  especially  towards  the  borders  of 
Durham,  there  is  a  larger  proportion  of  grass  land.  The 
Permian  area  of  south  Durham  is  very  largely  covered  by 
boulder-clay.  A  crop  peculiar  to  the  loams  of  the  Magnesian 
Limestone  is  liquorice,  which  is  grown  near  Pontefract. 

16—2 


244  THE   PERMIAN  AND  [CH. 

II.     THE  TRIASSIC  SYSTEM. 

The  Triassic  rocks  of  the  British  Isles  cover  a  very  large 
area,  especially  in  the  Midland  counties,  and  constitute  from 
the  agricultural  point  of  view  one  of  the  most  important  systems. 
As  before  remarked,  it  is  often  difficult  to  fix  the  dividing  line 
between  Permian  and  Trias,  owing  to  lithological  similarity 
and  rarity  or  absence  of  fossils.  Where  the  two  formations  are 
clearly  seen  in  contact  there  is  sometimes  a  slight  unconformity, 
and  the  Trias  overlaps  the  Permian  in  almost  all  directions; 
furthermore  the  upper  divisions  of  the  Trias  overlap  the  lower 
divisions  in  many  places,  resting  directly  on  Carboniferous  and 
older  rocks. 

The  Trias  of  Britain  is  divided,  mainly  on  lithological  grounds, 
into  three  groups,  namely : 

Rhaetic  series, 

Keuper  series, 

Bunter  series. 

The  two  lower  divisions  consist  of  conglomerates,  sandstones 
and  marls,  generally  of  a  bright  red  colour ;  the  Rhaetic  alone 
is  marine.  In  Germany  a  marine  or  inland-sea  limestone, 
known  as  the  Muschelkalk,  represents  the  middle  part  of  the 
Trias  and  in  the  eastern  Alps  the  whole  series  is  calcareous  and 
generally  dolomitic.  The  calcareous  facies  is  not  represented  in 
Britain,  except  to  a  very  small  extent  in  the  Rhaetic. 

Triassic  rocks  first  appear  on  the  south  coast  a  little  east 
of  the  mouth  of  the  Exe,  and  form  a  band  extending  due  north 
to  the  shores  of  the  Bristol  Channel ;  they  are  however  in  places 
concealed  under  Upper  Greenland  and  Chalk.  They  wrap 
round  the  flanks  of  the  Mendip  Hills  and  extend  in  a  gradually 
widening  band  up  the  Severn  valley ;  a  little  north  of  Worcester 
the  outcrop  suddenly  becomes  very  wide,  and  covers  a  large 
part  of  Worcestershire,  Warwickshire,  Leicestershire  and 
Staffordshire.  At  the  southern  end  of  the  Pennine  Chain  the 
outcrop  divides  into  two,  one  branch  occupying  almost  the 
whole  of  Cheshire  and  a  broad  strip  on  the  Lancashire  coast. 
After  a  short  interruption  at  Morecambe  Bay  the  Trias  again 
appears  in  Furness,  and  along  the  coast  of  Cumberland  as  far 


xii]  TRIASSIC   SYSTEMS  245 

as  Whitehaven.  From  Mary  port  it  spreads  out  widely  over 
the  plain  of  north  Cumberland  and  into  Dumfriesshire,  and 
also  accompanies  the  Permian  outcrop  up  the  Eden  valley.  In 
Scotland  there  are  one  or  two  very  small  scattered  patches  in 
Ayrshire  and  Arran,  along  the  coast  of  the  Moray  Firth,  and 
in  the  Inner  Hebrides;  Trias  is  also  seen  on  Belfast  Lough  and 
on  the  coast  of  Antrim.  The  eastern  branch  runs  due  north  from 
Nottingham,  forming  the  wide  plain  of  the  Trent  and  the  Vale 
of  York,  passing  out  to  sea  at  the  mouth  of  the  Tees.  It  will  be 
seen  therefore  that  the  Trias  forms  some  of  the  best  agricultural 
districts  in  the  whole  country. 

The  Bunter  beds  of  Devonshire  consist  of  two  members; 
about  80  feet  of  coarse  conglomerate  at  the  base  and  300  feet 
of  coarse  textured  red  sandstone  with  occasional  pebbles  above. 
The  pebble  beds  consist  of  well-rounded  or  oval  pebbles  up  to 
a  foot  in  diameter,  mainly  of  quartzite,  grit  and  Devonian 
limestone ;  the  pebbles  become  smaller  towards  the  north  while 
the  proportion  of  sandy  matrix  increases.  The  sandstones 
above  also  contain  occasional  pebbles,  and  the  whole  series 
must  have  been  laid  down  by  rapidly  moving  water,  probably 
in  a  great  river  flowing  from  the  north  or  north-west,  with  some 
addition  of  material  from  the  south-west,  since  part  of  the  sand 
grains  can  be  traced  to  the  metamorphic  areas  around  the 
granites  of  Devon  and  Cornwall. 

The  Keuper  beds  of  Devonshire  comprise  about  1200  feet 
of  red  and  green  marls,  with  occasional  beds  of  sandstone, 
layers  of  gypsum  and  casts  of  cubes  of  rock-salt.  The  whole 
series  must  have  been  formed  in  still  water  under  desert  con- 
ditions. The  uppermost  layers  for  about  100  feet  have  been 
bleached  by  water  percolating  from  above,  and  are  called  the 
Tea-green  Marls.  Above  this  comes  the  Rhaetic  marine 
series,  from  30  to  40  feet  thick.  The  latter  is  well  exposed  at 
several  points  on  the  shores  of  the  Bristol  Channel,  but  inland 
it  is  hardly  seen.  Around  the  Mendip  Hills  the  Bunter  is 
absent,  and  the  Keuper  is  partly  composed  of  wedge-shaped 
masses  of  a  peculiar  breccia,  generally  called  the  Dolomitic 
Conglomerate.  This  and  the  accompanying  sandstones  pass 
into  the  more  normal  Keuper  marls. 


246  THE   PERMIAN  AND  [CH. 

The  following  table  shows  the  succession  of  the  Trias  beds 
as  seen  in  Worcestershire.  It  may  be  taken  as  typical  of  the 
Midland  district: 

Feet 

Rhaetic  Beds      50 

-rr  (Red  marls  with  rock-salt  and  gypsum...         ...       1000 

(Red  sandstone  with  conglomerate  at  base      ...         300 
/-Upper  mottled  sandstone  ...         ...         ...         200 

Bunter     J  Pebble  beds  ...         300 

I  Lower  mottled  sandstone  ...         ...         ...         200 

In  general  terms  it  may  be  stated  that  the  Bunter  series  of 
the  Midlands  consists  of  coarse  sandstones  and  conglomerates, 
while  the  Keuper  is  composed  of  fine  sandstones  and  marls,  often 
with  rock-salt  and  gypsum.  The  Bunter  pebble  beds  thicken 
towards  the  north,  becoming  very  conspicuous  in  Staffordshire 
and  Cheshire,  where  they  form  some  poor  land,  as  in  Cannock 
Chase.  Towards  the  east  the  Bunter  thins  out,  and  in  the 
north-east  of  Warwickshire  this  division  is  absent.  However 
in  Leicestershire  and  Nottinghamshire  the  Bunter  comes  in 
again  and  forms  among  other  comparatively  poor  districts  the 
largely  uncultivated  area  of  Sherwood  Forest.  The  pebble 
beds  do  not  seem  to  extend  beyond  Doncaster.  In  the  Vale  of 
York  and  in  the  Tees  valley  the  Trias  is  so  largely  covered  by 
drift  that  little  is  known  about  it.  At  Middlesborough  a  thick 
bed  of  rock-salt  is  worked  in  the  Bunter,  all  the  other  British 
salt-beds  being  in  the  Keuper. 

In  Cheshire  the  total  thickness  of  the  Trias  is  very  great, 
as  follows: 

Feet 

[Marls  and  Upper  Sandstones  2000 

(Lower  Sandstone  (Waterstones)         400 
I  Upper  mottled  sandstone        ...         600 

Bunter      |  Pebble  beds         1000 

[Lower  mottled  sandstone        ...         400 

The  pebble  beds  here  are  much  the  same  as  in  Staffordshire 
but  thicker.  The  Waterstones  are  so  called  because  they  form 
a  good  water-bearing  horizon.  The  most  important  economic 
product  of  the  Trias  is  the  rock-salt  of  Cheshire,  which  forms 
two  beds  from  70  to  100  feet  thick. 


xn]  TBIASSIC   SYSTEMS  24.7 

The  Trias  beds  of  Lancashire  are  a  continuation  of  those  of 
Cheshire,  the  chief  difference  being  that  a  larger  proportion  of 
the  sandstones  are  yellow  instead  of  red.  The  Trias  of  Cumber- 
land and  the  Scottish  border  is  at  least  3000  feet  thick: 

Feet 

[Red  marl  and  gypsum  ...         900 

(Kirklinton  Sandstone  ...         500 


Bunter  Sandstone         ......       150° 

[Red  gypsiferous  marl    ...         ...         300 

The  most  important  member  is  the  St  Bees  Sandstone,  of  a 
dull  red  colour,  formerly  assigned  to  the  Permian.  It  is  well 
seen  on  the  coast  and  in  the  Eden  valley  north  of  Apple  by. 
The  softer  beds  of  the  Keuper  are  largely  covered  by  drift. 

The  outcrop  of  the  Rhaetic  beds  is  everywhere  so  narrow 
as  to  be  of  no  agricultural  importance. 

As  to  the  fossils  of  the  Trias  there  is  little  to  be  said.  It  is 
only  in  the  Rhaetic  that  a  few  species  of  marine  invertebrates 
are  found.  The  Bunter  and  Keuper,  being  terrestrial,  contain 
only  remains  of  reptiles,  amphibia,  fish  and  plants.  The 
reptilia  and  amphibia  are  commonly  represented  by  foot-prints 
preserved  in  the  fine  mud  of  the  lake  shores.  The  most 
important  fossils  of  the  British  Rhaetic  are  the  lamellibranchs, 
Avicula  contorla,  Protocardia  rhaetica  and  Pecten  Valoniensis, 
with  fish,  Acrodus  and  Ceratodus  and  reptiles,  Ichthyosaurus 
and  Plesiosaurus  .  In  the  Rhaetic  beds  of  the  Mendip  Hills  are 
found  remains  of  the  earliest  known  mammal,  Microlestes,  a 
small  marsupial. 

In  the  south  of  Staffordshire  the  Bunter  pebble  beds  form 
the  greater  part  of  the  heathy  and  forest-clad  area  known  as 
Cannock  Chase.  The  hills  rise  with  fairly  steep  sides  some  300 
or  400  feet  above  the  Trent.  Much  of  the  area  is  open  land 
with  heath  and  sparse  oaks  or  clusters  of  birch.  If  enclosed 
the  land  is  generally  under  the  plough,  but  a  large  part  of  it  is 
hardly  worth  cultivating,  the  soils  being  very  light  and  sandy 
or  gravelly.  The  Waterstones  and  Keuper  sandstones  are  thin 
reddish  or  white  sandstones  often  rising  up  as  small  hills  covered 
with  fairly  light  soil.  The  Keuper  marls  are  tough  red  clays 
with  occasional  green  bands,  yielding  heavy  soils;  these  form 


248  THE   PERMIAN  AND  [CH. 

pre-eminently  a  district  of  grass  land,  devoted  to  dairying, 
though  good  crops  of  corn  and  roots  can  be  grown.  In  general 
terms  it  may  be  said  that  the  Bunter  forms  either  land  too  poor 
to  be  cultivated  or  light  arable  soils,  while  the  Keuper  marl  is 
grass.  The  Keuper  sandstones  however  resemble  the  better 
land  of  the  Bunter. 

In  Cheshire  the  pebble  beds  yield  quite  a  good  light  soil  and 
carry  a  fair  amount  of  arable  land.  The  Lower  Keuper  sand- 
stones generally  form  high  escarpments,  as  at  Helsby  and  the 
Peckforton  Hills.  They  yield  a  light  fertile  soil,  which  has 
been  much  improved  in  the  past  by  "marling"  with  Keuper 
marl  or  boulder-clay;  this  process  was  very  costly,  but  such 
land  now  commands  high  rents.  There  is  some  waste  land  on 
this  formation  in  the  Wirral  peninsula.  The  Waterstones,  a 
mixture  of  thin-bedded  shales  and  red  and  green  sandstones  of 
very  fine  grain,  yield  rather  a  heavy  soil,  mostly  in  grass.  But 
the  leading  agricultural  characteristic  of  Cheshire  is  the 
prevalence  of  grass  land,  which  lies  on  the  Keuper  marls  and  on 
the  boulder-clays.  Both  of  these  formations  yield  clay  soils 
which  are  too  stiff  and  heavy  for  arable  cultivation,  but  form 
most  excellent  permanent  pasture,  admirably  adapted  to 
dairying  and  especially  to  the  making  of  cheese,  for  which 
this  county  is  noted.  In  the  south  of  Lancashire  the  Triassic 
strata  are  generally  buried  under  a  thick  cover  of  glacial  drift 
of  variable  character.  Here  and  there  the  Waterstones  come 
to  the  surface  and  form  a  rather  light  soil.  In  north  Cumberland 
and  in  the  Eden  valley  there  is  a  great  thickness  of  Trias,  for 
the  most  part  sandstones,  but  also  deeply  covered  by  drift  and 
alluvium,  which  almost  wholly  determine  the  character  of  the 
soils. 

The  soils  yielded  by  the  British  Trias  vary  greatly  in 
character,  including  some  of  the  poorest  as  well  as  some  of  the 
best  land  in  the  country.  Worst  of  all  are  the  Bunter  pebble 
beds,  which  are  sometimes  uncultivated,  heath-covered  or 
forest  land.  Of  the  other  subdivisions  the  sandstones  tend  to 
form  light  land,  often  fertile  and  well  suited  to  barley  and 
turnips,  while  the  marls  are  of  somewhat  heavy  character  with 
a  preponderance  of  permanent  pasture  and  dairy-farming. 


xii]  TRIASSIC   SYSTEMS  249 

One  of  the  most  fertile  districts  on  the  Trias  is  round  Taunton  in 
Somerset,  where  the  soil  is  a  light  free-working  loam,  rather 
deficient  in  lime,  but  admirably  adapted  for  barley ;  this 
crop  is  often  taken  three  times  in  a  five-year  rotation.  This 
district  lies  entirely  on  the  Keuper,  which  also  forms  the  base- 
ment of  the  great  alluvial  flats  (moors)  of  central  Somerset,  in 
the  lower  valleys  of  the  Tone  and  Parret  (Sedgemoor  and 
others).  This  country,  which  is  very  like  the  Fens  of  eastern 
England,  suffers  greatly  from  floods.  In  the  large  Triassic 
district  of  the  Midlands  the  soils  of  the  Bunter  are  generally 
light  and  sandy,  those  of  the  pebble  beds  being  extremely  so ; 
the  Keuper  sandstones  and  marls  carry  some  rich  soils,  often 
a  good  deal  modified  by  boulder-clay  and  drift.  In  Notting- 
hamshire the  Bunter  forms  light  sandy  soils,  generally  in  need 
of  lime,  but  when  well  treated  they  will  grow  good  crops  of 
barley,  oats  and  turnips.  Sherwood  Forest,  on  the  Bunter  pebble 
beds,  is  still  largely  uncultivated.  On  the  Waterstones  of  the 
Keuper  the  prevalent  soil  is  a  red,  slightly  calcareous,  clayey 
loam ;  a  bed  of  greenish  clay  at  the  base  forms  a  stiff  yellow  soil. 
The  Keuper  marls  form  a  stiff  red  calcareous  clay,  fertile  but 
difficult  to  work  and  needing  deep  drainage;  occasional  beds 
of  sandstone,  locally  called  skerries,  form  loamy  and  sometimes 
stony  soils.  As  before  stated  the  soils  of  the  Vale  of  York  can 
scarcely  be  regarded  as  truly  Triassic ;  they  are  derived  almost 
exclusively  from  river  alluvium  and  boulder-clay. 


CHAPTER  XIII 

THE  JURASSIC  SYSTEM 

We  now  come  to  what  is  admittedly  the  most  difficult  task 
of  the  agricultural  stratigraphist,  namely,  the  description  of 
the  rocks  composing  the  Jurassic  system.  These  rocks  show 
extreme  variation  of  lithological  character,  both  vertically  and 
horizontally.  In  any  given  locality  the  character  of  the 
sediment  deposited  during  this  period  changed  very  rapidly,  so 
that  the  succession  of  rock  types  is  complicated,  and  furthermore 
when  traced  from  place  to  place  the  rocks  composing  many  of 
the  subdivisions  show  remarkable  changes  of  facies;  conse- 
quently there  may  be  little  or  no  resemblance  in  the  soils 
yielded  by  the  same  formation  in  different  localities.  For  this 
reason  the  information  afforded  by  geological  maps  is,  in  respect 
of  this  system,  particularly  misleading  and  must  of  necessity 
be  supplemented  by  local  knowledge.  Hence  it  is  very  difficult 
to  give  in  a  reasonable  compass  a  clear  account  of  this  most 
important  formation. 

The  distribution  of  the  Jurassic  system,  taken  as  a  whole, 
is  particularly  simple ;  it  forms  a  broad  band  stretching  across 
the  country  from  Dorsetshire  to  Yorkshire,  the  only  inter- 
ruption being  that  due  to  the  overlap  of  the  Cretaceous  in  the 
East  Riding  of  Yorkshire,  on  the  western  flank  of  the  Wolds. 
There  are  two  small  outliers  in  Shropshire  and  Cumberland 
respectively  and  a  few  small  and  unimportant  outcrops  in  the 
north-east  of  Ireland  and  on  both  sides  of  the  northern  part  of 
Scotland. 

It  is  unfortunate  that  the  nomenclature  of  this  system  is  in 
a  somewhat  confused  state.  It  was  originally  described  as  the 


CH.  xm]  THE   JURASSIC   SYSTEM  251 

Oolitic  series,  from  the  prevalence  of  oolite  limestone  in  the 
middle  members.  At  a  later  stage  this  name  was  used  by  some 
writers  for  the  whole  system  as  at  present  defined,  by  others 
for  a  part  of  it  only.  Hence  this  name  should  be  avoided. 
The  following  table  shows  the  divisions  as  at  present  adopted 
by  most  authorities : 

/Purbeck  Series. 

Portland  Series. 
Upper  Jurassic  4  Kimeridge  Series. 

Corallian  Series. 
\Oxfordian  Series. 

(Bathonian  or  Great  Oolite  Series.   ' 
Middle  Jurassic    s-r,   •     •  T  £    •      f\  m.    a    • 

(Bajocian  or  Inferior  Oolite  feenes. 

Lower  Jurassic        Lias. 

Over  a  large  part  of  the  country  the  Portland  and  Purbeck 
beds  are  absent,  and  the  thickness  of  the  different  subdivisions 
varies  greatly.  In  Dorset,  where  the  succession  is  most  complete, 
the  thickness  is  about  3500  feet,  but  in  the  Midlands  and 
Yorkshire  it  is  much  less.  Nevertheless  owing  to  the  low  dip 
the  outcrop  is  very  wide  even  in  these  districts,  especially  just 
south  of  the  Wash.  The  Jurassic  rocks  form  a  region  of  the 
most  varied  relief,  ranging  from  the  steep  and  rugged  hills  and 
moorlands  of  north  Yorkshire  (Cleveland  and  Hambleton  Hills), 
the  Cotteswolds  and  the  hills  of  Dorset,  to  the  level  plains  of  the 
eastern  Midlands  (Huntingdonshire,  Cambridgeshire  and  south 
Lincolnshire).  Almost  the  whole  of  the  great  plain  of  the  Wash 
is  underlain  by  Jurassic  clays,  although  they  are  concealed 
from  view  by  alluvium  and  peat.  It  is  perhaps  worthy  of 
mention  that  the  comparatively  soft  rocks  of  the  Lias  form 
some  of  the  highest  and  steepest  clifls  on  the  English  coast. 
This  is  the  case  both  in  Yorkshire  and  in  Dorset,  and  is  due 
largely  to  the  fact  that  the  Lias  is  overlain  by  much  harder 
rocks,  forming  the  cap  of  the  cliff. 

So  far  as  it  is  possible  to  generalize  on  the  lithology  of  the 
Jurassic  system  as  a  whole,  it  may  be  said,  contrary  to  the 
general  impression,  to  be  a  clay  formation.  It  is  true  that 
sandstones  and  limestones  are  very  abundant,  appearing  in 
some  localities  to  be  the  dominant  rock-type,  but  this  appear- 
ance is  in  part  deceptive,  since  the  hard  rocks  make  a  great 


252  THE   JURASSIC   SYSTEM  [CH. 

show  in  cliffs,  escarpments  and  quarries,  while  the  softer 
members  form  low  ground  and  are  largely  concealed  by  super- 
ficial deposits.  Limestones  are  most  strongly  developed  in  the 
south-western  district,  clays  in  the  Midlands  and  sandstones 
in  Yorkshire. 

The  general  stratigraphy  of  the  Jurassic  system  has  been 
studied  with  great  minuteness,  perhaps  more  so  than  in  any 
other  formation.  It  is  found  that  the  variations  of  facies  and  the 
occasional  gaps  in  the  system  are  largely  due  to  repeated  small 
crust-movements,  following  the  lines  of  axes  of  folding  initiated 
at  much  earlier  periods ;  these  are  called  posthumous  movements. 
These  small  disturbances  were  not  much  in  evidence  during  the 
Liassic  period,  but  reached  their  maximum  in  the  Middle 
Jurassic,  afterwards  again  becoming  less  conspicuous,  till  at 
the  close  there  was  a  sudden  recrudescence,  leading  in  many 
places  to  a  strong  unconformity  as  the  base  of  the  Cretaceous. 

A  problem  of  some  interest  is  the  source  of  the  material 
composing  the  Jurassic  rocks.  The  dominant  lithological  types 
are  limestones  and  blue  clays  and  shales,  with  sands  only  in  the 
shallower  water.  Many  of  the  argillaceous  rocks  are  very 
bituminous,  some  being  capable  of  combustion ;  this  abundance 
of  carbon  is  of  some  significance  with  regard  to  their  origin, 
since  it  suggests  derivation  from  Coal-measures.  The  material 
for  the  very  abundant  limestones  might  well  also  be  derived 
from  the  Carboniferous  Limestone.  Much  of  the  material  must 
have  come  from  land  to  the  north,  as  indicated  by  the  prevalence 
of  estuarine  conditions  in  Yorkshire  in  Middle  Jurassic  times 
and  indications  of  a  shore-line  in  this  direction  in  the  Oxfordian 
and  Corallian.  In  the  south  material  must  have  been  derived 
from  the  Devonian  peninsula  and  the  Mendip  Hills,  as  well  as 
from  the  broad  ridge  of  old  land  that  then  clearly  joined  the 
Mendip  region  to  the  Carboniferous  of  Belgium,  the  so-called 
London  plateau,  whose  existence  has  of  late  years  been  proved 
in  so  many  deep  borings.  Against  this  old  land  Triassic, 
Jurassic  and  Lower  Cretaceous  strata  abutted  and  overlapped, 
till  it  was  finally  submerged  in  the  middle  of  the  Cretaceous 
period. 


xm]  THE   JURASSIC   SYSTEM  253 

I.     LOWER  JURASSIC.     THE  LIAS 

The  Lias  forms  a  well-defined  series  with  distinctive  char- 
acters, differing  markedly  from  the  succeeding  Middle  Jurassic 
strata.  It  is  also  a  good  deal  more  uniform,  when  traced  across 
country,  than  any  of  the  higher  divisions.  The  thickness  also 
is  fairly  uniform,  from  800  to  1000  feet. 

The  Lias  is  essentially  a  clay  formation ;  in  the  lower  part 
especially  there  are  a  good  many  calcareous  bands,  but  these 
usually  alternate  in  thin  layers  with  shale  or  clay.  At  certain 
horizons  there  are  bands  of  ironstone  of  great  economic  value; 
the  chief  of  these  being  in  the  Cleveland  district  of  Yorkshire. 

As  a  matter  of  practical  convenience  the  Lias  is  usually 
divided  into  Lower,  Middle  and  Upper,  but  these  divisions  do 
not  correspond  to  any  clearly  marked  differences,  either  litho- 
logical  or  palaeontological.  In  some  districts  the  demarcation 
of  them  is  somewhat  uncertain.  In  a  more  strictly  scientific 
way  the  subdivisions  are  effected  by  means  of  ammonite-zones, 
this  being  the  characteristic  fossil-group  of  the  whole  series. 
About  fourteen  principal  zones  are  recognized,  with  numerous 
sub-zones.  By  means  of  these  the  succession  in  distant  areas 
can  be  correlated. 

The  Lower  Lias  as  seen  on  the  Dorset  coast  near  Lyme 
Regis  consists  of  about  500  feet  of  grey  shales,  clays  and  marls, 
with  many  bands  of  limestone,  especially  near  the  base.  Most 
of  the  beds  of  shale  and  clay  are  more  or  less  calcareous,  and 
fossils  are  abundant;  the  bands  of  limestone  are  often  only  a 
foot  or  two  thick,  alternating  with  shale.  The  origin  of  the 
limestone  bands  is  supposed  to  be  mechanical,  that  is  to  say, 
they  consist  of  calcareous  deposits  formed  by  denudation  of 
some  older  limestone,  probably  the  Lower  Carboniferous.  The 
Middle  Lias  is  somewhat  more  sandy,  and  about  350  feet  thick, 
while  the  Upper  Lias  of  the  south-western  district  is  very  thin, 
certainly  not  more  than  70  feet  and  possibly  much  less.  Round 
the  Mendip  Hills  and  in  Glamorgan  the  Lias  is  very  thin,  being 
in  fact  a  shore-deposit,  the  whole  series  at  the  same  time 
becoming  much  more  calcareous,  and  in  South  Wales  there  are 
some  massive  white  limestones.  When  followed  up  the  Severn 


254  THE  JURASSIC  SYSTEM  [CH. 

valley  the  series  again  becomes  thicker  and  more  argillaceous. 
In  Warwickshire  the  Lower  Lias  is  mostly  clay,  but  the  upper 
part  of  the  middle  division  consists  of  the  well-known  Marlstone, 
a  ferruginous  and  sandy  limestone  that  caps  much  of  the  high 
ground,  rising  to  over  700  feet  at  Edge  Hill.  The  Upper  Lias  of 
this  area  is  mainly  clay  or  shale,  and  generally  about  150  feet 
thick. 

Between  Rugby  and  Lincoln  the  Lias  maintains  the  same 
general  character,  but  becomes  somewhat  thicker;  the  Lower 
Lias  alone  near  Grantham  is  about  700  feet  thick,  and  at  the 
top  of  it  is  a  valuable  bed  of  ironstone,  largely  worked  in 
Lincolnshire.  The  Marlstone  is  also  worked  for  iron  near 
Grantham.  The  Upper  Lias  near  Lincoln  consists  of  a  series 
of  shales  with  calcareous  concretions  and  thin  limestones,  the 
total  thickness  being  about  100  feet. 

In  Yorkshire  the  total  thickness  of  the  Lias  is  about  1100 
feet.  The  Lower  Lias  is  as  usual  an  alternation  of  shales  and 
thin  limestones,  about  700  feet  thick,  but  the  middle  division 
is  much  more  sandy;  moreover  it  contains  some  extremely 
important  seams  of  ironstone.  The  Cleveland  main  seam  at 
Eston  reaches  the  great  thickness  of  25  feet  of  solid  ironstone ; 
besides  this  there  are  several  other  seams,  giving  rise  to  one  of 
the  largest  iron  industries  in  the  world. 

The  Upper  Lias  consists  of  about  220  feet  of  shale ;  at  one 
time  the  upper  part  was  largely  used  for  the  manufacture  of 
alum,  but  this  is  now  an  extinct  industry  in  this  district.  The 
jet  trade  is  also  moribund;  jet  is  a  kind  of  bituminized  fossil 
wood,  found  near  the  middle  of  the  Upper  Lias. 

The  Liassic  outliers  of  Shropshire,  Cumberland,  Scotland 
and  Ireland  are  lithologically  very  similar  to  the  main  outcrop ; 
the  total  area  is  small,  and  that  in  Cumberland  is  entirely 
covered  by  drift.  Those  in  Scotland  and  Ireland  are  of  no 
agricultural  interest,  but  a  thick  bed  of  ironstone  in  the  Middle 
Lias  of  Raasay  will  doubtless  be  of  great  commercial  importance 
some  day. 

Since  the  Lias  is  in  the  main  a  clay-formation  it  generally 
forms  somewhat  low  ground.  There  are  however  important 
exceptions.  Both  in  Dorset  and  in  Yorkshire  it  often  forms 


xiii]  THE  JURASSIC  SYSTEM  255 

the  lower  parts  of  high  cliffs,  where  the  soft  Lias  has  been 
protected  from  denudation  by  a  capping  of  some  harder  rock.. 
Again  the  Middle  Lias  (Marlstone)  of  the  Midlands  is  a  very 
hard  ferruginous  limestone,  which,  although  of  no  great  thick- 
ness, forms  a  prominent  escarpment  in  Warwickshire  (Edge 
Hill),  Northamptonshire  and  in  the  south  of  Lincolnshire,  near 
Grantham.  However  the  Lower  Lias,  which  is  the  thickest 
division,  almost  everywhere  forms  either  a  low  plain  or  the 
floor  of  deep  valleys  excavated  through  the  harder  beds  above. 
Most  of  the  patches  of  higher  ground  scattered  about  the 
Liassic  plain  of  the  Midlands  are  outliers  of  hard  ironstone  or 
limestone  belonging  to  the  Middle  Jurassic  series. 

The  soils  yielded  by  the  Lias  must  on  the  whole  be  described 
as  fertile,  and  there  are  few  barren  or  uncultivated  tracts  on 
this  formation.  The  Lias  generally  forms  fairly  low  ground 
and  consequently  it  tends  to  be  covered  by  accumulations  of 
alluvium  (e.g.  Sedgemoor  in  Somerset)  and  by  drift  in  the  more 
northern  half  of  the  country.  Generally  speaking  the  limestones 
and  ironstones  yield  loamy  soils,  forming  good  arable  land, 
while  the  more  purely  argillaceous  beds  are  rather  heavy  and 
as  a  rule  are  very  largely  under  permanent  pasture.  The 
Lower  Lias  limestones  of  the  south-west  of  England  form 
fairly  rich  brown  loams  of  free-working  character  and  yielding 
good  crops  of  corn  and  roots.  In  particular  the  Marlstone  of 
the  Middle  Lias  forms  some  rich  land  in  Dorset  and  south 
Somerset.  The  clays  of  the  Lower  and  Middle  Lias  in  this  area 
are  often  cold  and  wet,  forming  heavy  land  in  the  valleys, 
mostly  under  grass,  and  in  places  carrying  productive  orchards 
for  cider-making.  These  heavy  lands  are  largely  used  for 
dairy-farming  and  especially  for  cheese-making  on  permanent 
pasture.  The  upper  part  of  the  Middle  Lias  and  the  Upper 
Lias  are  lighter  in  character,  yielding  loams  rather  than  clays, 
with  more  arable  land.  The  well-known  fruit-growing  district 
round  Evesham  lies  largely  on  Lias.  The  soil  here  is  heavy  and 
not  naturally  rich,  but  it  is  subjected  to  an  intensive  system  of 
cultivation  and  is  thus  made  very  productive.  In  Warwick- 
shire and  Northamptonshire  clays  predominate  and  carry  the 
famous  pastures  of  those  shires,  arable  land  being  rare  unless 


256  THE   JURASSIC   SYSTEM  [CH. 

the  Lias  is  covered  by  drift.  The  Marlstone  and  part  of  the 
.  Upper  Lias  are  more  ferruginous  and  calcareous,  yielding  good 
red  loams  in  Rutland  and  south  Lincolnshire,  and  all  over  this 
area  the  soils  of  the  Lias  plains  are  to  a  considerable  extent 
modified  by  rainwash  carried  down  from  the  higher  ground  of 
Northampton  Sands  and  other  calcareous  and  iron-bearing 
rocks.  Thus  their  texture  and  composition  is  often  considerably 
improved. 

From  Leicestershire  northwards  the  outcrop  of  the  Lias  is 
almost  always  masked  by  a  thick  covering  of  glacial  drift  or  by 
some  kind  of  alluvium ;  hence  it  becomes  more  difficult  to  give 
any  general  account  of  the  soils.  Where  free  from  such  superficial 
accumulations  their  general  character  is  much  the  same  as  in 
the  driftless  area  of  the  south-western  counties,  with  pre- 
dominating pasture  land  on  the  heavy  clays  and  good  arable 
loams  on  the  limestones  and  ironstones.  In  Lincolnshire  the 
Lias  forms  strong  land,  suited  to  wheat,  beans  and  mangolds, 
though  much  of  it  is  in  pasture  largely  devoted  to  grazing  and 
milk-farming.  In  north  Yorkshire  the  Lias  outcrops  mostly  at 
the  bottom  of  deep  valleys,  with  steep  sloping  sides  capped  by 
the  estuarine  sandstones  of  the  Bajocian;  hence  the  Lias  soils 
are  much  obscured  by  rainwash  and  drift.  Where  free  from 
such  covering  they  generally  form  heavy  clays,  mostly  in 
permanent  pasture.  The  Lias  plain  north  and  west  of  the 
Cleveland-Hambleton  escarpment  is  so  thickly  covered  by  drift 
that  the  underlying  rock  is  a  negligible  factor  in  determining 
the  character  of  the  soils. 

II.     THE  MIDDLE  JURASSIC 

The  truly  marine  conditions  of  the  Lias  appear  to  have 
come  to  a  somewhat  abrupt  conclusion ;  everywhere  the  water 
became  much  shallower,  clays  and  shales  giving  place  to  lime- 
stones and  sandstones;  in  some  places  there  is  a  distinct 
discontinuity  and  in  a  few  localities  it  is  possible  to  prove 
considerable  denudation  of  the  Lias  before  the  deposition  of  the 
next  succeeding  series.  This  is  specially  noticeable  in  north- 
east Yorkshire,  where  in  some  places  as  much  as  100  feet,  or 


xm]  THE  JURASSIC  SYSTEM  257 

even  more,  of  Lias  has  been  carried  away,  the  hollows  in  its 
surface  being  filled  up  by  coarse  sandstone  and  impure  ironstone ; 
throughout  the  counties  of  Lincoln,  Rutland  and  Northampton 
there  is  also  an  unconformity  at  this  horizon,  and  the  base  of 
the  upper  series  is  formed  by  a  pebble  bed  containing  rolled 
fossils  derived  from  the  Lias. 

The  succession  of  Middle  Jurassic  strata,  as  seen  in  the  south- 
western counties,  may  be  generalized  as  follows : 

/Cornbrash. 

Forest  Marble  and  Bradford  Clay. 

Bathonian  or  Great  Oolite  beries  •{  /-,      ,   ^  v,          ,  0,         n  , ,  01   . 

Great  Oolite  and  Stonesfield  Slate. 

(Fullers'  Earth. 

T  t    •      /**&•.«•      ( Inferior  Oolite. 
Baiocian  or  Inferior  Oolite  Series  i  ,...,,     ,  0 

^Midford  Sands. 

The  above  succession  generally  holds,  although  there  are 
local  variations  and  some  of  the  divisions  are  discontinuous. 
The  chief  difficulty  arises  from  the  fact  that  the  basal  sands, 
though  almost  everywhere  present,  are  not  always  at  the  same 
horizon,  as  indicated  by  the  fossils.  However  interesting  from 
the  purely  scientific  standpoint,  this  is  of  little  practical 
importance.  In  Dorset  the  Midford  sands  are  from  150  to  180 
feet  thick ;  in  Somerset  they  enclose  beds  of  shelly  limestone. 
These  pass  up  gradually  into  the  limestones  of  the  Inferior 
Oolite  proper,  which  are  only  about  10  feet  thick  on  the  coast, 
but  near  Yeovil  they  increase  to  about  50  feet.  The  lowest 
member  of  the  Bathonian  in  Dorset  is  the  Fullers'  Earth, 
which  attains  a  thickness  of  400  feet;  it  is  a  soft  marly  clay, 
in  part  capable  of  being  used  for  "fulling"  cloth  (i.e.  removing 
grease)  and  for  clearing  oils.  Next  comes  the  Forest  Marble, 
a  series  of  shelly  and  flaggy  limestones  about  100  feet  thick. 
This  is  succeeded  normally  by  the  Cornbrash,  whose  general 
characters  will  be  described  later. 

Perhaps  the  most  typical  development  of  the  calcareous 
facies  of  the  Middle  Jurassic  rocks  is  that  seen  in  the  well-known 
agricultural  district  of  the  Cotteswold  Hills.  The  general 
succession  is  as  follows : 


R.A.G.  17 


258  THE  JURASSIC  SYSTEM  [CH. 

Feet 

/Cornbrash        15 

I  Forest  Marble  00-100 

Great  Oolite  Group      -.'Great  Oolite  40-100 

Istonesfield  Slate        10-15 

\Fullers'  Earth  40-80 

(Ragstones        20-  40 

Inferior  Oolite  Group   J  Freestones  and  Pea  Grit     ...  35-200 

(Cotteswold  Sands      40-120 

The  Cotteswold  sands,  though  closely  resembling  the 
Midford  sands,  clearly  belong  to  a  higher  stratigraphical  horizon. 
They  are  thickest  just  north  of  the  Mendip  Hills  and  thin  out 
towards  the  north-east;  most  of  the  limestones,  on  the  other 
hand,  are  thicker  in  this  direction.  Although  the  limestones 
have  been  divided  by  specialists  into  a  great  number  of  minor 
groups,  such  elaboration  is  unnecessary  for  the  present  purpose. 
The  whole  Middle  Jurassic  succession  of  the  Cotteswold  region 
above  the  sands  may  be  briefly  and  generally  described  as 
consisting  mainly  of  limestones,  often  markedly  oolitic  or 
shelly,  often  flaggy  in  structure,  and  with  occasional  inter- 
calations of  shale  or  clay,  such  for  example  as  the  so-called 
Bradford  Clay  at  the  base  of  the  Forest  Marble  series;  this 
band,  though  well  known  and  conspicuous,  is  only  10  feet  thick. 
The  limestones  vary  a  good  deal  in  character,  and  show  every 
gradation  from  one  type  to  another.  The  more  compact  beds 
form  excellent  building  stone,  as  for  example  the  famous  Bath 
stone. 

When  followed  into  Oxfordshire  and  Northamptonshire  the 
Bajocian  series  becomes  entirely  sandy  and  ferruginous,  and 
partly  of  estuarine  origin,  but  the  Bathonian  succession  is  very 
similar  to  that  seen  in  Gloucestershire.  At  the  base  of  the 
whole  succession  are  some  valuable  beds  of  ironstone,  exten- 
sively worked  in  Northamptonshire  and  Rutland.  When 
followed  towards  the  north  the  estuarine  facies  becomes  more 
and  more  dominant,  the  only  important  marine  bed  being 
the  Lincolnshire  Limestone;  this  forms  a  valuable  building- 
stone,  well-known  quarries  being  those  at  Clipsham,  Ketton  and 
Ancaster.  This  series  forms  the  remarkable  long,  straight 
escarpment  that  extends  throughout  Lincolnshire  in  a  due  north 


xin]  THE  JURASSIC  SYSTEM  259 

and  south  direction.  The  city  of  Lincoln  is  situated  at  the 
point  where  it  is  cut  through  by  the  river  Witham.  At  the 
base  of  the  Lincolnshire  Limestone  in  the  south  of  the  county 
is  a  layer  of  thin-bedded  sandy  limestone  much  used  for  roofing 
purposes,  the  so-called  Collyweston  slate;  this  is  of  course 
flagstone,  and  not  true  slate.  The  Bathonian  series  of  Lincoln- 
shire consists  partly  of  clays  of  various  colours,  the  representative 
of  the  Great  Oolite  limestone  being  only  about  20  feet  thick. 
The  Cornbrash  at  the  top  is  as  usual  (see  below).  The  so-called 
"heath"  land  of  Lincolnshire  lies  on  these  formations.  The 
soils  are  light  and  well  adapted  to  the  growth  of  barley,  roots 
and  seeds,  also  the  rearing  and  fattening  of  sheep. 

In  the  East  Riding  of  Yorkshire  the  Middle  Jurassic  is  for 
the  most  part  concealed  by  the  overlap  of  the  Cretaceous,  but 
in  the  North  Riding  there  is  an  extensive  development  of  beds 
of  this  age,  for  the  most  part  estuarine  in  origin,  with  a  total 
thickness  of  about  650  feet.  There  are  one  or  two  thin  cal- 
careous beds  representing  the  limestones  of  the  Midlands  and 
the  succession  is  terminated  by  the  Cornbrash.  The  estuarine 
beds  of  Yorkshire  are  a  very  variable  succession  of  sandstones 
and  shales,  with  plant  remains  and  occasional  thin  seams  of 
coal,  the  whole  showing  a  strong  resemblance  to  the  Coal- 
measures  on  a  small  scale.  At  the  base  is  the  Dogger,  which 
is  in  some  places  a  ferruginous  sandstone,  but  in  other  places 
a  valuable  seam  of  ironstone.  The  whole  series  is  well  seen  in 
the  cliffs  between  Scarborough  and  Saltburn,  and  forms  the 
capping  of  the  Cleveland  and  Hambleton  Hills,  which  rise  to 
a  height  of  over  1400  feet.  A  great  part  of  the  outcrop  is  open 
moorland,  intersected  by  deep  valleys  cut  through  into  the 
Lias. 

The  Cornbrash  is  one  of  the  most  curiously  persistent  beds 
in  the  whole  Jurassic  succession,  hardly  varying  in  thickness 
or  in  character  from  one  end  to  the  other  of  its  outcrop.  It 
consists  of  light-coloured  earthy  or  rubbly  limestone,  often  rich 
in  fossils  and  generally  from  5  to  15  feet  in  thickness,  though 
locally  it  may  be  somewhat  thicker.  The  limestone  is  said 
to  be  unusually  rich  in  phosphoric  acid,  at  any  rate  in  the 
south  of  England,  and  this  may  partly  account  for  the  fertility 

17—2 


260  THE  JURASSIC  SYSTEM  [CH. 

of  the  soil  yielded  by  it.  In  the  north  it  is  more  ferruginous  and 
sometimes  oolitic,  the  soils  being  less  fertile. 

One  of  the  most  noticeable  features  of  the  calcareous  facies 
of  the  Middle  Jurassic  strata  is  the  abundance  of  oolitic  and 
pisolitic  limestones,  from  the  prevalence  of  which  the  whole 
formation  was  formerly  known  as  the  Oolitic  system.  Lime- 
stones of  this  type  are  formed  in  shallow  but  clear  water,  often 
in  the  near  neighbourhood  of  coral  reefs,  where  calcareous 
organisms  are  abundant  and  the  water  easily  becomes  com- 
paratively rich  in  calcium  carbonate.  The  mode  of  origin  of 
such  rocks  has  been  discussed  in  an  earlier  chapter  (see  p.  91). 
Many  of  the  beds  of  oolite  form  excellent  building  stone.  The 
beds  of  clay  indicate  either  a  temporary  deepening  of  the  water 
or  an  influx  of  mud  from  some  distant  source.  The  estuarine 
strata  generally  show  a  rapid  alternation  of  sandstones  and 
shales,  and  present  every  indication  of  having  been  formed  as 
delta  deposits  at  the  mouths  of  large  rivers.  Occasional  thin 
marine  bands  show  temporary  incursions  of  the  sea,  probably 
due  to  slight  local  depression.  The  ironstones  are  almost 
invariably  formed  by  metasomatic  replacement  of  limestones 
as  described  on  p.  97. 

The  fossils  of  the  Middle  Jurassic  strata  naturally  vary  in 
character  according  to  the  conditions  of  formation  of  the  sedi- 
ment. The  limestones  and  other  marine  beds  contain  a  remark- 
ably abundant  marine  fauna,  specially  rich  in  ammonites. 
Lamellibranchs,  gastropods,  brachiopods  and  corals  are  also 
very  abundant,  and  many  of  the  limestones  consist  almost 
exclusively  of  more  or  less  broken  shells.  It  has  been  found 
possible  to  divide  the  marine  facies  into  zones  by  means  of 
their  characteristic  fossils,  chiefly  ammonites,  thus: 

[Cornbrash  Amm.  macrocephalus. 

T,  ,,      .         I  Forest  Marble  Waldheimia  diqona. 

Bathoman    {  _.      A    _.  .._,  -_    .  Tr  7j  . 

Great  Oolite  Nennaea  Voltzi. 

^Fullers'  Earth  Amm.  subcontractus. 

/  (Amm.  Parkinsoni. 

Upper  Limestones        \Amm    Humphriesianus. 

Bajocian      -j  Lower  Limestones  Amm.  Murchisonae. 

I  Amm.  opalinus. 
ttriford  Sands  Jmm. 


xm]  THE  JURASSIC  SYSTEM  261 

This  table  applies  to  the  south-western  succession;  the 
correlation  of  the  estuarine  facies  presents  some  difficulties, 
ftut  it  seems  clear  that  the  Lincolnshire  Limestone  belongs  to 
the  zone  of  Amm.  Murchisonae,  while  the  Scarborough  lime- 
stone, the  chief  calcareous  bed  in  Yorkshire,  belongs  to  the 
Humphriesianus  zone.  All  the  beds  between  this  and  the 
Cornbrash  are  estuarine. 

Owing  to  the  varying  lithological  character  of  the  beds 
composing  the  Middle  Jurassic  series  they  offer  unequal 
resistance  to  denudation,  and  as  a  consequence  the  topography 
of  the  area  occupied  by  these  strata  shows  much  variation.  In 
the  south-west  of  England  the  limestone  bands  are  the  hardest 
and  form  strongly  marked  escarpments,  with  the  steep  face 
towards  the  north-west  and  a  gentle  dip-slope  to  the  south-east ; 
this  for  example  is  the  general  structure  of  the  Cotteswold 
Hills.  The  Bath  Oolite  and  the  Forest  Marble  also  form  con- 
spicuous escarpments  in  Somerset  and  Dorset,  often  rising  into 
flat-topped  hills,  whereas  those  composed  of  Midford  Sands  are 
commonly  conical  in  shape.  The  Fullers'  Earth  and  other  beds 
of  clay  form  valleys  between  the  escarpments.  When  followed 
through  the  Midland  counties  the  Bajocian  and  Bathonian 
limestones  and  sandstones  usually  constitute  fairly  high  ground, 
often  in  distinct  escarpments,  strongly  contrasting  with  the 
plains  of  the  Lias  on  the  one  hand  and  of  the  Oxford  Clay  on 
the  other.  As  already  mentioned  there  are  scattered  over  the 
Midland  counties  many  outliers  of  these  rocks  resting  on  Lias. 
Owing  to  differential  erosion  and  faulting  the  distribution  of 
this  series  is  almost  everywhere  very  complex  at  the  surface, 
and  consequently  the  soils  show  great  variety  within  short 
distances. 

Owing  to  the  rapid  variations,  both  vertical  and  horizontal, 
in  the  character  and  composition  of  the  Middle  Jurassic  rocks, 
it  is  almost  impossible  to  give  any  general  account  of  the  soils 
yielded  by  them.  Furthermore  over  the  northern  half  of  their 
outcrop,  from  Northamptonshire  to  Yorkshire,  the  matter  is 
complicated  by  a  widespread  cover  of  glacial  drift  of  varying 
character.  In  general  terms  it  may  be  said  that  the  clays  and 
shales  yield  heavy  soils,  difficult  to  cultivate  and  largely  under 


262  THE  JURASSIC  SYSTEM  [CH. 

permanent  pasture,  devoted  to  stock-feeding  and  dairy- farming ; 
the  limestones  on  the  other  hand  carry  light  loamy  soils,  often 
rather  stony,  and  eminently  adapted  to  barley  and  turnips; 
they  are  rarely  strong  enough  to  give  the  heaviest  yields  of 
wheat,  although  this  crop  is  extensively  grown.  Such  soils  are 
liable  to  suffer  from  drought  in  dry  seasons,  and  water  supply 
is  often  a  difficulty.  The  sandstones  of  the  Midlands  often  give 
rise  to  light  soils  of  good  quality,  but  in  the  moorland  district 
of  Yorkshire  they  are  largely  uncultivated  and  covered  with 
heather  and  peat ;  owing  to  the  high  elevation  they  are  devoted 
to  Blackface  sheep.  The  character  of  the  soils  lying  on  some 
of  the  more  distinctive  subdivisions  may  be  very  briefly  men- 
tioned. The  Midford  and  Cotteswold  sands  carry  light  soils 
but  are  limited  in  their  extent.  The  limestones  of  the  Inferior 
Oolite  form  what  are  often  called  brashy  soils,  that  is,  fairly 
light  loams,  often  rather  stony  and  well  adapted  to  corn  growing, 
especially  barley,  and  also  suitable  for  turnips.  These  limestones, 
form  the  soils  of  the  well-known  Cotteswold  sheep  district. 
The  Fullers'  Earth  on  the  other  hand  is  very  heavy  and  wet, 
and  is  mostly  under  grass.  The  Great  Oolite  limestone  forms- 
a  rather  thin  "brashy"  soil,  suitable  for  turnips  and  barley,  but 
the  clay  part'  of  this  division  is  a  heavy  soil.  The  Cornbrash  as- 
its  name  implies  is  a  good  corn-growing  soil,  but  in  the  northern 
part  of  its  outcrop  it  is  not  so  fertile  as  in  the  south-west  of 
England.  The  Lincolnshire  Limestone  forms  a  light  and  not  very 
productive  soil,  while  the  estuarine  clays  of  the  region  between 
Northamptonshire  and  the  Humber  are  rather  stiff  and  of  poor 
quality.  In  the  south  of  Yorkshire  the  outcrop  of  this  series 
is  very  narrow ;  further  north  it  expands  again  and  forms  the 
large  area  of  high-lying  ground  known  as  the  North  Yorkshire 
moors,  whose  character  has  already  been  sufficiently  indicated. 

III.     THE  UPPER  JURASSIC 

The  rocks  composing  the  Upper  Jurassic  series  show  less 
variation  than  those  of  the  Middle  Jurassic,  but  even  here  most 
of  the  major  subdivisions  comprise  at  least  two  distinct  facies. 
The  distribution  of  these  rock-types  is  however  easily  explained 


xm]  THE   JURASSIC   SYSTEM  263 

on  the  assumption  that  the  British  area  then  consisted  of  a 
sea  basin,  deep  over  the  Midland  counties,  shallower  towards 
the  south-west  and  the  north-east,  with  in  all  probability  a 
shore-line  at  no  great  distance  in  both  directions.  In  the  later 
stages  land  encroached  on  the  sea  in  the  south  of  England, 
while  in  the  north  the  sea  became  deeper. 

The  generalized  classification  of  the  Upper  Jurassic,  as 
usually  adopted,  together  with  the  prevailing  character  of  the 
rocks,  is  shown  in  the  following  table: 


South  of  England     Midlands 

North  of  England 

Purbeckian  ~i 
Portlandian  j 

Limestone 

Absent 

Absent  ? 

Kimeridgian    .  .  . 

Clay 

Clay 

Clay. 

Corallian 

Limestone 

Clay 

Limestone. 

Oxfordian 

Clay 

Clay 

Sandstone. 

Hence  it  appears  that  this  is  largely  an  argillaceous 
formation,  the  limestones  and  sandstones  being  local  and 
subordinate. 

The  Oxfordian  subdivision  takes  its  name  from  the  Oxford 
Clay,  which  in  the  stretch  of  country  between  Oxford  and  the 
Wash  forms  practically  its  whole  thickness.  However  both  in 
the  south-west  of  England  and  in  Yorkshire  the  lower  part  con- 
sists of  sandstones,  often  with  a  calcareous  cement,  known  as 
the  Kellaways  Rock ;  this  is  a  shallow- water  facies  of  the 
lower  zones  of  the  Oxfordian,  indicating  approach  to  shore-lines 
in  both  directions.  In  Dorset  the  Kellaways  beds  are  about 
70  feet  thick,  but  they  become  thinner  to  the  north-east  and 
disappear  in  Bedfordshire.  In  north  Yorkshire  the  Kellaways 
rock  sets  in  again  and  attains  in  the  moorland  district  a  thickness 
of  about  100  feet,  forming  some  very  bold  features.  The 
Oxford  Clay,  which  forms  the  surface  rock  of  a  very  large  area 
in  the  eastern  Midlands,  is  a  stiff,  bluish,  greenish  or  grey  clay, 
becoming  brown  or  yellow  at  the  surface  owing  to  oxidation. 
Sometimes  it  is  rather  shaly,  sometimes  quite  without  signs  of 
bedding,  breaking  only  with  a  conchoidal  fracture  into  irregular 
lumps.  There  are  occasional  harder  calcareous  bands  and  lines 
of  concretions,  while  in  some  places  large  crystals  of  gypsum 


264  THE  JURASSIC  SYSTEM  [CH. 

(selenite)  are  abundant.  Fossils  are  numerous,  especially 
ammonites,  belemnites  and  large  oysters  (Gryphaea  dilatata  and 
allied  forms).  The  ammonites  are  usually  preserved  in  iron 
pyrites. 

The  Oxfordian  may  be  conveniently  divided  by  means  of 
its  ammonites  into  three  zones: 

Zone  of  Ammonites  cordatus, 
Zone  of  Ammonites  ornatus, 
Zone  of  Ammonites  calloviensis. 

These  are  sometimes  further  divided  into  sub-zones.  Amm. 
calloviensis  seems  to  be  confined  to  the  sandy  Kellaways  facies ; 
and  is  scarcely  known  where  the  whole  series  is  argillaceous, 
as  in  Huntingdonshire.  In  some  localities  remains  of  reptiles 
are  very  abundant. 

Owing  to  the  great  uniformity  of  the  Oxford  Clay  it  is 
unnecessary  to  describe  its  distribution  in  detail;  from  the 
Dorset  coast  to  the  Humber  it  varies  only  in  the  extent  to  which 
it  is  replaced  by  the  Kellaways  facies;  in  North  Yorkshire 
more  than  half  the  formation  is  Kellaways  rock,  and  the 
rest  is  a  grey  sandy  shale  with  few  fossils.  This  however 
mostly  forms  uncultivated  moorland,  and  is  of  little  im- 
portance. 

In  the  Corallian  series  the  existence  of  two  distinct  facies 
is  very  conspicuous;  both  in  Dorset  and  in  Yorkshire  this 
division  consists  of  an  alternation  of  calcareous  sandstones  and 
limestones,  generally  oolitic;  in  the  Midland  counties  these 
are  replaced  by  a  clay,  very  similar  to  the  Oxford  Clay  and  often 
confused  with  it,  as  for  example  on  the  published  maps  of  the 
Geological  Survey.  Owing  to  the  great  difference  in  the 
hardness  of  the  two  rock  types  the  topography  of  the  Corallian 
country  shows  strong  variations.  In  Dorset  and  Wiltshire 
and  in  Yorkshire  it  forms  elevated  ground  with  conspicuous 
escarpments,  while  in  the  Midlands  it  forms  part  of  the  great 
plain  of  the  Wash  drainage  basin. 

The  Corallian  series  of  the  north  of  England,  as  developed 
in  the  neighbourhood  of  Pickering,  shows  a  rather  complicated 
series  of  sandstones  and  limestones  with  an  occasional  shaly 
band: 


xiii]  THE  JURASSIC  SYSTEM  265 

Feet 

Upper  Calcareous  Grit       40 

Upper  Limestone  and  Coral  Rag  \  _~ 
Coralline  Oolite        ...         ...           J 

Middle  Calcareous  Grit      80 

Lower  Limestones  ...         ...         ...  100 

Lower  Calcareous  Grit                   ...  130 


400 

In  the  above  table  the  succession  is  simplified  as  much  as 
possible  and  the  thicknesses  given  are  the  maximum  for  each 
division.  The  Calcareous  Grits  are  massive  yellowish  sandstones 
with  a  calcareous  cement,  often  weathered  into  striking  tabular 
and  columnar  forms.  The  Lower  Calcareous  Grit  forms  a  very 
well-marked  escarpment  facing  north,  and  caps  many  of  the 
tabular  hills  of  the  moorland  district. 

The  limestones  vary  a  good  deal  in  character ;  some  beds  are 
oolitic  or  pisolitic,  while  others  are  very  rich  in  corals  and  shells. 
Part  of  the  lower  limestone  series  is  to  a  certain  extent  silicified 
and  converted  into  chert,  especially  near  Filey.  The  term 
Coral  Rag  is  commonly  used  to  describe  the  massive  shelly 
non-oolitic  varieties  of  limestone,  some  of  which  appear  to  be 
true  coral  reefs. 

In  Dorset  and  Wiltshire  the  general  succession  is  similar; 
the  Lower  Calcareous  Grit  appears  to  comprise  also  the  repre- 
sentatives of  the  Lower  Limestones  and  the  Middle  Grit:  that 
is,  the  sandy  facies  extends  higher  in  the  series.  The  upper 
limestones  are  very  similar  to  those  in  Yorkshire,  but  near 
Westbury  they  are  largely  converted  into  a  valuable  bed  of 
iron  ore.  A  few  miles  east  of  Oxford  the  limestones  come  to 
an  end  and  from  Buckinghamshire  to  the  Humber  the  clay 
facies  alone  is  found,  except  for  a  small  coral  reef  at  Upware, 
near  Cambridge. 

The  argillaceous  representative  of  the  Corallian,  the  Ampthill 
Clay,  was  laid  down  in  fairly  deep  but  muddy  water  and  consists 
of  a  dark  bluish  grey  clay  with,  in  some  places,  a  band  of  ferru- 
ginous oolitic  limestone  at  the  base.  The  total  thickness  is 
somewhat  uncertain;  it  probably  does  not  anywhere  exceed 
50  feet,  and  may  be  much  less.  Most  of  the  outcrop  of  the 
Ampthill  Clay  is  covered  by  drift  and  alluvium.  It  is  almost 


266  THE  JURASSIC  SYSTEM  [cm, 

invariably  mapped  as  Oxford  Clay,  but  can  be  distinguished 
in  the  field  from  this  formation  by  the  fact  that  the  fossils  are 
preserved  in  calcite  instead  of  iron  pyrites.  Agriculturally  it 
is  almost  exactly  similar  to  the  Oxford  Clay  and  the  line  of 
demarcation  is  uncertain,  both  above  and  below. 

There  can  be  no  doubt  as  to  the  geographical  conditions 
prevailing  in  Britain  in  Corallian  times.  The  limestones  and 
sandstones  of  the  north  and  south-west  were  laid  down  near  to 
shore-lines,  in  clear  water  with  strong  currents  and  abundant 
animal  life;  a  warm  climate  favoured  the  growth  of  reef- 
building  corals.  In  the  central  part  of  the  basin  the  sea  was 
deeper  but  muddy,  and  therefore  unfavourable  for  calcareous, 
organisms.  The  mud  was  probably  derived  from  the  denudation 
of  an  ancient  land  now  hidden  under  the  Cretaceous  rocks  of 
south-eastern  England. 

The  Kimeridgian  series  is  a  much  more  uniform  formation 
than  the  Corallian;  it  consists  entirely  of  argillaceous  strata, 
the  Kimeridge  Clay;  this,  though  varying  considerably  in 
thickness  in  different  parts  of  the  country,  shows  very  constant 
lithological  characters.  It  consists  of  dark  grey  or  black  clay, 
usually  more  or  less  shaly,  with  bands  of  calcareous  nodules 
and  crystals  of  selenite.  Occasionally  there  are  to  be  seen  thin 
layers  of  grey  or  whitish  impure  limestone  or  hardened  marl. 
In  some  places  the  clay  is  very  bituminous  and  fossils  are 
commonly  abundant,  though  as  a  rule  not  well  preserved. 

The  Kimeridge  Clay  on  the  Dorset  coast  is  about  1000  feet 
thick,  but  it  decreases  rapidly  towards  the  north-east;  from 
Oxfordshire  to  the  Wash  there  is  only  about  100  feet;  this  is 
partly  to  be  accounted  for  by  denudation  before  the  formation 
of  the  overlying  Cretaceous  rocks,  but  in  the  main  it  is  due  to 
a  real  diminution  in  the  original  thickness  on  passing  towards 
the  centre  of  the  basin  of  deposition.  In  Lincolnshire  the 
thickness  increases  again  to  about  600  feet;  in  Yorkshire, 
owing  to  concealment  by  drift  and  alluvium,  the  thickness  is 
uncertain,  but  is  probably  not  less  than  in  Lincolnshire. 

Owing  to  its  softness  the  Kimeridge  Clay  nearly  always 
forms  low  ground.  In  the  south-west  of  England  the  surface 
exposures  are  not  large,  owing  to  the  overlap  of  the  Cretaceous, 


xm]  THE  JURASSIC  SYSTEM  267 

and  in  the  central  portion  the  outcrop  is  narrow  on  account  of 
the  small  thickness  of  the  clay  itself.  The  Kimeridge  Clay 
certainly  underlies  a  large  part  of  the  Fenland  and  in  Lincoln- 
shire it  forms  the  eastern  half  of  the  broad  plain  between  the 
Cliff  and  the  Wolds.  In  Yorkshire  it  forms  the  basement  of 
at  any  rate  the  greater  part  of  the  Vale  of  Pickering,  although 
there  are  no  exposures  and  the  clay  is  deeply  buried  in  drift 
and  alluvium.  It  can  however  be  seen  on  the  coast  near 
Speeton. 

Rocks  of  the  Portland  and  Purbeck  series  cover  only  a 
very  small  surface  area  in  the  British  Isles.  Where  best 
developed  on  the  Dorset  coast  the  Portlandian  series  comprises 
two  subdivisions,  the  Portland  Sand  below  and  the  Portland 
Stone  above.  The  sandy  beds  vary  in  thickness  from  40  to 
160  feet  but  are  of  little  interest  or  importance.  The  Portland 
Stone  series  has  a  total  thickness  of  about  100  feet  and  includes 
the  famous  building-stone,  together  with  some  silicified  cherty 
beds.  The  building- stone  is  a  very  white  oolitic  limestone, 
passing  into  a  white  compact  rock  very  like  Chalk,  even  con- 
taining flint  or  chert  in  nodules.  The  whole  group  thins 
towards  the  north ;  in  the  Vale  of  Wardour,  west  of  Salisbury, 
it  is  only  about  100  feet  thick,  and  at  Swindon  and  near  Oxford 
almost  the  whole  of  the  series  is  sandy.  In  Buckinghamshire 
the  lower  part  of  the  Portlandian  is  represented  by  the  Hartwell 
Clay,  with  sands  and  thin  limestones  above. 

Rocks  of  the  Purbeckian  series  are  only  known  with 
certainty  to  come  to  the  surface  in  the  Isle  of  Portland,  the 
Isle  of  Purbeck,  the  Vale  of  W^ardour,  and  a  small  patch  of 
undefined  extent  in  the  middle  of  the  Wreald.  The  latter  is 
usually  omitted  from  geological  maps.  In  Dorset  and  Wiltshire 
they  consist  of  thin  limestones,  shales  and  marls,  formed  under 
varying  conditions,  marine,  estuarine  and  fresh- water;  some 
black  earthy  layers  are  actually  old  surface  soils,  with  stumps 
of  trees  still  rooted  in  them.  Many  of  the  beds  are  very 
rich  in  fossils,  and  the  shell-limestones  are  quarried  under 
the  name  of  Purbeck  Marble.  The  total  thickness  is  about 
400  feet.  In  the  Vale  of  Wardour  the  thickness  is  only 
about  100  feet,  and  the  area  exposed  is  small.  Near  Swindon 


268  THE  JURASSIC  SYSTEM  [CH.  xm 

and  in  Buckinghamshire  are  some  thin  beds  with  fresh-water 
fossils  that  may  be  of  Purbeck  age. 

Portland  and  Purbeck  beds  are  not  again  seen  south  of  the 
Wash,  and  it  is  still  uncertain  whether  they  are  represented  by 
any  part  of  the  Spilsby  Sandstone  in  Lincolnshire.  In  York- 
shire the  uppermost  Jurassic  and  Lower  Cretaceous  strata  are 
represented  by  the  Speeton  Clay,  a  thin  black  clay  very  like 
the  Kimeridge,  only  exposed  on  the  coast.  This  may  underlie 
the  southern  part  of  the  Vale  of  Pickering,  as  it  is  said  to  have 
been  seen  in  borings  at  the  foot  of  the  Wolds. 

The  Oxford  Clay  forms  some  of  the  heaviest  soils  in  the  whole 
of  the  British  Isles.  They  are  difficult  and  expensive  to  work, 
and  owing  to  the  flatness  of  most  of  the  ground,  nearly  always 
in  need  of  draining.  In  Huntingdonshire  and  the  Fenland, 
where  the  outcrop  is  widest,  there  is  generally  a  thick  covering 
of  drift  and  alluvium,  and  the  true  surface  of  the  Oxford  Clay 
is  often  below  sea-level.  The  soils  of  the  Corallian  division 
naturally  show  much  variation ;  the  Calcareous  Grits  of  York- 
shire form  high  ground  bearing  a  light  sandy  soil,  much  of  which 
is  moorland.  The  limestones  which  form  the  southern  part  of 
the  hilly  region  stretching  for  some  40  miles  west  of  Scarborough, 
and  also  the  western  part  of  the  range  bounding  the  Vale  of 
Pickering  on  the  south  yield  on  the  contrary  good  loamy  soils, 
often  rather  light  and  stony  but  carrying  good  crops  of  corn. 
Where  this  rock-type  reappears  in  the  south-west  of  England 
from  Oxford  to  Dorset  the  soils  are  again  of  very  similar  char- 
acter. The  Ampthill  Clay  of  the  Midlands  is  almost  exactly 
like  the  Oxford  Clay,  but  is  mostly  covered  by  drift.  The 
Kimeridge  Clay  forms  almost  uniformly  a  heavy  rather  cold 
soil,  covered  by  drift  and  alluvium  in  the  Fenland,  Lincolnshire 
and  the  Vale  of  Pickering.  Further  south  it  forms  the  soil  of 
the  important  dairy-farming  district  of  the  Vale  of  Aylesbury 
and  of  the  Vale  of  White  Horse,  and  reappears  in  some  force  in 
the  Vale  of  Wardour  in  Wiltshire.  The  clays  of  the  Upper 
Jurassic,  where  free  from  drift,  form  low-lying  damp  districts 
which  are  mostly  under  permanent  pasture  and  specially 
suitable  for  dairying. 


CHAPTER  XIV 

THE   CRETACEOUS  SYSTEM 

Towards  the  end  of  Jurassic  times  a  series  of  earth-move- 
ments began  which  produced  important  results  in  English 
stratigraphy.  In  the  south  of  England  the  estuarine  and 
fresh- water  type  of  deposit  continued  for  some  time ;  in  York- 
shire on  the  other  hand  the  sea  remained  deep  and  in  it  was 
formed  the  Speeton  Clay,  which  may  be  regarded  as  a  continua- 
tion of  the  Kimeridge  Clay.  Over  the  tract  of  country  from 
Oxfordshire  to  the  Wash  the  disturbance  was  most  strongly 
felt  and  produced  a  well-marked  unconformity,  accompanied  by 
considerable  denudation  of  the  deposits  just  formed.  In  some 
part  or  other  of  this  tract  the  base  of  the  Cretaceous  rests  on 
each  of  the  subdivisions  of  the  Jurassic  from  the  Oxford  Clay 
upwards.  Between  the  Wash  and  the  Humber  there  is  not 
much  indication  of  unconformity,  but  it  is  very  doubtful 
whether  the  succession  is  complete.  Not  only  are  the  higher 
beds  of  the  Jurassic  absent  in  the  eastern  Midlands,  but  the 
lower  divisions  of  the  Cretaceous  were  never  deposited.  There 
was  a  broad  ridge  of  land,  with  its  highest  part  in  Bedfordshire 
or  Cambridgeshire,  separating  two  seas;  this  ridge  was 
gradually  submerged  during  the  earlier  part  of  Cretaceous  time 
and  the  successive  marine  deposits  overlapped  one  another 
from  each  side  towards  the  centre ;  by  the  beginning  of  Upper 
Cretaceous  time  the  ridge  was  completely  submerged,  as  well 
as  the  old  land  of  the  London  plateau,  which  was  undoubtedly 
the  source  of  much  of  the  material  of  the  Lower  Cretaceous 
strata. 

The  Cretaceous  system  falls  naturally  into  two  divisions, 
Lower  and  Upper;  owing  to  the  great  variability  in  thickness 


270  THE   CRETACEOUS  SYSTEM  [CH. 

and  character  of  the  Lower  Cretaceous  it  is  difficult  to  devise 
a  classification  and  nomenclature  of  general  application.  The 
names  usually  applied  are  taken  from  the  development  as  seen 
on  the  coast  of  Kent  and  Sussex,  but  they  are  strictly  of  local 
application  only,  being  unsuited  to  the  rocks  as  seen  in  Devon, 
Yorkshire  or  Ireland.  These  facts  can  be  expressed  in  the 
general  statement  that  the  Cretaceous  system  shows  much 
variation  of  facies,  and  that  the  southern  facies  is  usually 
accepted  as  typical.  On  this  basis  the  classification  is  as  follows  : 

f  Chalk. 
Upper  Cretaceous    •{  „ 

tGault. 


Lower  Cretaceous 

^Wealden. 

The  local  distribution  and  variations  of  facies  in  each  of 
these  divisions  will  be  described  separately. 

The  Wealden  series.  This  series,  which  is  sometimes 
called  the  Neocomian,  is  somewhat  limited  in  its  distribution 
in  Britain.  The  estuarine  and  fresh-water  facies,  the  Wealden 
proper,  is  found  only  south  of  the  Thames.  Over  the  area 
between  the  Thames  and  the  Wash  it  is  absent,  but  comes  in 
again  in  Lincolnshire  and  Yorkshire  as  a  marine  facies.  It  is 
only  in  the  Weald  district  itself  that  this  formation  covers  any 
considerable  area. 

In  Kent,  Surrey  and  Sussex  the  Wealden  series  is  divided 
on  lithological  grounds  into  two  parts,  the  Hastings  Sands  below 
and  the  Weald  Clay  above.  The  topography  and  soils  that 
prevail  on  these  two  formations  differ  a  good  deal.  The 
Hastings  Sands  form  the  central  part  of  the  Wealden  district, 
rising  in  Crowborough  Beacon  to  a  height  of  nearly  800  feet. 
The  maximum  thickness  is  about  1000  feet  and  three  divisions 
are  recognized;  the  Wadhurst  Clay,  in  the  middle,  about  150 
feet  thick,  separating  two  main  masses  of  sand  and  sandstone,  the 
Ashdown  Sand  below  and  the  Tunbridge  Sand  above.  The  soils 
yielded  by  this  formation  are  somewhat  variable  in  character, 
but  mostly  poor.  Some  of  the  sands  are  so  fine  in  texture  that 
in  practice  they  are  almost  as  heavy  as  clays.  They  are  mottled 
yellow  and  green  a  little  below  the  surface  and  there  is  a  strong 
tendency  to  the  formation  of  ironstone  concretions.  The  soils 


xiv]  THE   CRETACEOUS  SYSTEM  271 

are  always  markedly  deficient  in  carbonate  of  lime.  A  large 
proportion  of  the  High.  Weald  country  on  the  Hastings  Sands  is 
wooded  and  there  is  a  considerable  area  of  unenclosed  land  in 
Ashdown  Forest.  The  lower  slopes  are  mostly  rather  poor 
grass  and  it  is  only  on  the  floors  of  the  valleys  in  the  eastern 
part  of  the  area  that  any  good  farming  is  found.  As  a  rule 
neither  turnips  nor  barley  are  grown.  In  the  better  districts 
hops  are  cultivated  to  a  certain  extent.  The  chief  industry  is 
stock-farming  on  grass  land,  with  dairying  near  the  railway 
lines. 

The  Weald  Clay  has  a  maximum  thickness  of  about  900  feet 
in  the  west,  thinning  to  the  east.  It  consists  of  blue  and  brown 
clays  with  occasional  layers  of  shelly  limestone  and  sand.  The 
fossils  are  chiefly  fresh- water  shells.  In  strong  contrast  to  the 
sandy  strata  above  and  below  it,  the  Weald  Clay  forms  a  flat 
plain  which  is  a  striking  feature  in  a  contoured  map  of  the 
district.  This  is  an  area  of  extraordinarily  heavy  soil,  and  is 
now  nearly  all  grass  and  woodland.  There  is  very  little  arable 
land  and  on  this  neither  turnips  nor  barley  are  grown,  though 
wheat,  beans  and  mangolds  do  fairly  well.  The  grass  land  is 
poor  in  quality  and  is  chiefly  devoted  to  the  breeding  of  Romney 
Marsh  sheep  and  Sussex  cattle.  The  small  acreage  under  hops 
is  rapidly  decreasing. 

The  Weald  Clay  forms  a  stiff  greasy  clay  with  nodules  and 
concretions  of  iron  oxide  in  the  subsoil.  These  are  sometimes 
almost  numerous  enough  to  form  a  pan,  and  to  require  breaking 
up  with  a  crowbar  before  trees  can  be  planted.  Some  samples 
of  these  soils  contain  as  little  as  10  per  cent,  of  sand,  the  rest 
being  fine  silt  and  clay.  The  most  notable  chemical  character- 
istic of  all  Weald  soils  is  the  deficiency  in  calcium  carbonate. 

The  Weald  Clay  is  also  exposed  to  a  small  extent  in  the 
Isle  of  Wight  and  in  Dorset,  near  Swanage,  where  it  is  over  2000 
feet  thick  but  the  base  is  not  seen.  It  consists  of  variously 
coloured  clays  with  occasional  sand  beds;  the  upper  part  is 
chiefly  grey  shale.  Owing  to  the  steep  dip  the  outcrop  in  Dorset 
is  narrow.  In  the  Vale  of  Wardour  in  Wiltshire  the  Wealden 
series  is  reduced  to  about  100  feet  of  clay  and  beyond  this  no 
beds  of  this  age  are  seen  south  of  the  Wash.  In  Lincolnshire 


272  THE   CRETACEOUS  SYSTEM  [CH. 

the  Wealden  series  appears  to  be  represented  by  the  whole  or 
part  of  the  Spilsby  Sandstone;  possibly  also  the  Claxby  iron- 
stone and  Tealby  Clay  may  belong  here,  but  the  correlation  of 
the  Lower  Cretaceous  rocks  of  Norfolk  and  Lincolnshire  is  so 
uncertain  that  nothing  definite  can  be  said  on  this  point.  In 
Yorkshire  the  equivalent  is  to  be  found  in  the  lower  and  middle 
zones  of  the  Speeton  Clay.  In  neither  case  is  the  area  of  the 
outcrop  large  enough  to  render  the  question  of  any  agricultural 
importance. 

The  Lower  Greensand  series.  Where  most  complete,  in 
the  south-eastern  counties,  this  series  consists  of  a  bed  of  clay 
at  the  base,  the  Atherfield  Clay,  followed  by  a  considerable 
thickness  of  sand  and  sandstone.  The  whole  series  is  marine, 
in  contrast  to  the  fresh-water  and  estuarine  deposits  of  the 
Wealden.  In  the  Isle  of  Wight  the  total  thickness  is  about 
800  feet,  but  towards  the  north  it  becomes  much  thinner  owing 
to  the  gradual  dying  out  of  the  lower  divisions;  in  parts  of 
Cambridgeshire  it  appears  to  be  absent  altogether.  In  west 
Norfolk  and  in  Lincolnshire  lower  beds  again  come  in,  but  in 
Yorkshire  the  Speeton  Clay  facies  of  deposition  still  continues. 
The  Lower  Greensand  between  Dorset  and  the  Wash  affords  an 
excellent  example  of  unconformable  overlap,  the  successive  beds 
gradually  extending  further  and  further  from  either  side  over 
a  ridge  of  Jurassic  rocks  whose  summit  lay  on  the  borders  of 
Bedfordshire  and  Cambridgeshire.  It  was  not  until  Upper 
Cretaceous  times  that  this  ridge  was  completely  submerged. 
Hence  it  follows  that  south  of  the  Humber  the  Lower  Greensand 
consists  entirely  of  shallow- water  deposits,  for  the  most  part  coarse 
sands  with  occasional  pebble  beds.  The  name  Greensand  refers 
to  the  occurrence  in  many  places  of  a  conspicuous  amount  of 
glauconite,  imparting  a  green  colour  to  the  unweathered  sands ; 
this  green  colour  disappears  when  the  sand  is  at  or  near  the 
surface  of  the  ground,  owing  to  oxidation  of  the  iron.  Agri- 
culturally the  Lower  Greensand  is  important  as  a  soil  former  in 
two  areas,  namely,  (a)  Kent,  Surrey  and  Sussex ;  (6)  along  the 
outcrop  from  Dorset  to  the  Humber.  Each  of  these  will  be 
dealt  with  separately. 

The   south-eastern   area.     The   Lower   Greensand   forms   a 


xiv]  THE   CRETACEOUS   SYSTEM  273 

strip  of  generally  elevated  country,  of  varying  width,  all  round 
the  Weald,  except  on  the  coast.  On  the  north  the  outcrop  runs 
westwards  from  Folkestone  to  Farnham ;  then  it  turns  due  south 
to  Midhurst,  whence  it  runs  east-south-east  at  the  foot  of  the 
South  Downs  to  Eastbourne.  Leith  Hill,  965  feet,  chiefly 
composed  of  Lower  Greensand,  is  the  highest  point  in  the 
south-east  of  England,  and  the  well-known  high  ground  about 
Hindhead  and  Haslemere  is  also  on  this  formation.  The  greatest 
width  of  the  outcrop,  near  Godalming,  is  about  12  miles,  but 
the  southern  belt  is  much  narrower. 

In  this  district  four  subdivisions  are  recognized,  namely: 

Folkestone  beds  Green  and  grey  sand  and  sand-       80  feet. 

stone  with  bands  of  chert 

Sandgate  beds  Sand,  clay  and  Fullers'  Earth       70      „ 

Hythe  beds  Greenish  yellow  sand  and  hard    200      „ 

grey      calcareous      sandstone 

(Kentish  Rag) 
Atherfield  Clay  Blue  clay  with  marine  fossils         60      „ 

The  Atherfield  Clay  is  a  stiff  blue  clay,  weathering  brown, 
very  like  the  Weald  Clay,  but  distinguished  from  it  by  the 
presence  of  marine  fossils.  Though  naturally  very  heavy 
the  soil  is  modified  by  downwash  of  sand  from  the  beds  above, 
which  form  higher  ground;  hence  the  soil  is  a  fertile  loam. 

The  Hythe  beds  carry  very  different  soils  in  different  parts. 
In  Kent  the  greater  part  is  a  free-working,  though  rather  stony 
loam,  admirably  adapted  to  hops  and  fruit.  Around  Sevenoaks 
hops  of  the  best  quality  are  largely  grown.  Some  argillaceous 
beds  give  a  rather  heavy  soil  in  places.  The  hard  Kentish  Rag 
forms  a  good  building  stone,  though  it  cannot  be  dressed 
smooth.  West  of  Redhill  the  soils  become  almost  pure  sand 
and  are  almost  entirely  woodland  or  open  uncultivated  commons 
covered  with  heather,  gorse,  birch  and  pine.  This  is  now  a 
very  favourite  residential  district,  owing  to  its  high  elevation, 
dry  soil  and  beautiful  scenery.  The  southern  outcrop,  though 
narrow,  carries  some  good  barley  soils  near  Midhurst. 

The  Sandgate  beds  are  of  finer  texture  and  form  a  rather 
heavier  soil,  which  is  partly  in  pasture  in  the  valleys.  Some 
thin  beds  of  impervious  clay  cause  the  soils  here  to  be  rather 

B.A.G.  18 


274  THE   CRETACEOUS   SYSTEM  [CH. 

wet.  Beds  of  Fullers'  Earth  are  worked  at  Nutfield.  Near 
Godalming  a  bed  of  calcareous  rock,  the  Bargate  stone,  forms  a 
light  free- working  loam,  which  is  well  adapted  for  arable  sheep- 
farming.  The  Rother  valley  in  west  Sussex  is  an  area  of  fertile 
arable  land,  also  largely  under  sheep. 

The  Folkestone  Sand  is  much  coarser  in  texture  than  the 
other  divisions,  giving  rise  to  very  poor  sandy  soils,  largely 
uncultivated  and  forming  heaths  and  commons,  sometimes 
covered  by  sour  black  peaty  soil  where  a  pan  has  been  formed. 
In  recent  times  a  good  deal  of  land  has  been  planted  with 
Scotch  fir  and  Austrian  pine. 

To  sum  up,  it  may  be  said  that  the  great  majority  of  the 
Lower  Greensand  soils  are  very  light,  and  some  of  them  are 
almost  pure  sand  and  therefore  useless  for  cultivation.  Only  in 
the  lowest  division,  the  Atherfield  Clay,  is  there  any  notable 
amount  of  heavy  land.  The  most  strongly  marked  peculiarity 
in  all  the  soils  is  the  almost  complete  absence  of  lime,  and 
humus  is  generally  very  deficient.  Only  in  a  few  low -lying 
areas  is  there  any  really  good  soil,  doubtless  largely  due  to  an 
admixture  of  rainwash  and  alluvium. 

The  Lower  Greensand  series  occupies  a  considerable  area  in 
the  southern  part  of  the  Isle  of  Wight  and  here  reaches  its 
maximum  thickness,  about  800  feet.  Above  the  Atherfield 
Clay,  some  80  feet  thick,  are  ferruginous  sands  and  sandstones, 
with  a  good  many  marine  fossils.  The  uppermost  part,  locally 
known  as  the  Carstone,  appears  from  recent  researches  to  belong 
really  to  the  Gault  series,  and  is  therefore  on  a  higher  horizon 
than  the  Carstone  of  Norfolk  and  Lincolnshire. 

Western  and  northern  outcrops.  The  Lower  Greensand  is  seen 
in  many  places  along  the  Cretaceous  outcrop  from  the  Dorset 
coast  to  the  Wash,  but  it  is  not  by  any  means  continuous,  being 
sometimes  absent  altogether  or  overlapped  by  the  Gault. 
There  is  a  narrow  strip  of  it  in  Wiltshire,  from  near  Devizes  to 
Faringdon,  and  a  patch  south  of  Oxford,  where  it  is  mainly 
occupied  by  Nuneham  Park ;  a  larger  and  more  important  strip 
extends  from  Leighton  Buzzard  to  the  neighbourhood  of 
Cambridge ;  including  the  well-known  areas  of  Woburn  Sands, 
Sandy  and  Potton  in  Bedfordshire.  Beyond  Cambridge  the 


xiv]  THE   CRETACEOUS   SYSTEM  275 

outcrop  becomes  narrow  and  discontinuous,  forming  the  caps 
of  some  of  the  higher  lands  in  the  Fens,  especially  the  Isle  of 
Ely.  Even  here  however  it  is  largely  concealed  by  drift.  In 
Norfolk  again  there  is  an  interesting  outcrop  of  this  series 
extending  from  Downham  Market  to  Hunstanton.  In  Lincoln- 
shire this  series  outcrops  over  a  considerable  area  on  the  south 
and  west  of  the  Chalk  Wolds,  but  in  Yorkshire  it  is  represented 
only  by  the  uppermost  part  of  the  Speeton  Clay.  The  Lower 
Greensand  in  Bedfordshire  forms  a  considerable  area  of  very 
light  sandy  soils,  especially  around  Woburn  Sands,  Ampthill, 
Sandy  and  Potton.  A  very  large  part  of  this  is  in  parks  and 
woodlands;  the  rainwash  from  the  Lower  Greensand  escarp- 
ment when  mixed  with  river  alluvium  forms  a  soil  eminently 
suitable  for 'vegetable  growing,  as  in  the  Ivel  valley  near  Sandy 
and  Biggleswade ;  from  here  the  London  markets  are  largely 
supplied.  Somewhat  further  east  the  outcrop  of  the  Lower 
Greensand  forms  soil  suitable  for  fruit-growing,  especially  at 
Histon  and  Cottenham,  north-east  of  Cambridge. 

The  small  patches  capping  the  Isle  of  Ely  and  other  slightly 
elevated  areas  in  the  Fens  are  often  in  their  turn  covered  by 
glacial  gravels  and  sands,  the  soils  differing  considerably  from 
the  black  soils  of  the  lower  levels.  From  King's  Lynn  to  Hun- 
stanton is  a  strip  of  sandy  land,  with  much  heather  and  bracken, 
largely  woodland,  but  carrying  some  good  light  soils,  which 
however  are  often  much  modified  by  boulder-clay  and  other 
glacial  deposits.  This  district  is  admirably  adapted  for  game 
preserving,  as  at  Sandringham. 

The  Gault  series.  The  strata  lying  between  the  Lower 
Greensand  and  the  Chalk  show  much  variation  in  character  in 
different  parts  of  the  country.  As  seen  on  the  coast  of  Kent, 
at  Folkestone,  almost  the  whole  series  consists  of  pale  grey  clay1 
with  many  fossils.  Further  west,  in  Surrey  and  Hampshire, 
the  upper  part  consists  of  sands  and  sandstones,  often  containing 
glauconite.  This  sandy  facies,  the  so-called  Upper  Greensand, 

1  Gault  is  a  Cambridgeshire  word  signifying  originally  any  stiff  grey  clay, 
including  the  Oxford,  Ampthill  and  parts  of  the  Kimeridge  Clays,  as  well  as 
the  true  Gault.  It  is  still  used  in  this  sense  by  well  sinkers  and  others,  whence 
has  arisen  much  confusion. 

18—2 


276  THE   CRETACEOUS  SYSTEM  [CH. 

increases  in  thickness  towards  the  west,  at  the  expense  of  the 
clay  and  in  Dorset  it  comprises  almost  the  whole  formation, 
only  a  few  feet  of  clay  remaining  at  the  base  near  Lyme  Regis. 
In  Devonshire  and  Somerset  the  whole  is  Greensand.  When 
the  outcrop  is  followed  from  Somerset  towards  the  north-east 
a  similar  change  is  seen  in  reverse  order,  the  clay  becoming 
thicker  and  thicker  till  the  Greensand  dies  out  altogether  in 
Bedfordshire.  Between  this  point  and  the  borders  of  Norfolk 
the  whole  series  is  clay,  thinning  towards  the  north;  near 
King's  Lynn  a  further  change  occurs  and  at  Hunstanton  the 
whole  is  represented  by  3  or  4  feet  of  a  curious  red  limestone. 
This  reappears  beyond  the  Wash  and  continues  through  Lincoln- 
shire to  the  Yorkshire  coast,  being  thicker  than  in  Norfolk  and 
paler  in  colour;  north  of  the  Wash  it  is  called  the  Red  Chalk. 

The  best  exposure  of  the  Gault  is  at  Folkestone,  where  it 
is  about  100  feet  thick,  but  it  thickens  towards  the  west.  It  is 
a  stiff  bluish  or  grey  clay  with  occasional  seams  of  glauconitic 
sand  and  many  phosphatic  nodules.  Fossils  are  very  abundant 
and  preserved  in  a  peculiar  manner,  often  retaining  the  pearly 
lustre  and  iridescence  of  the  shell. 

The  Gault  clay  forms  a  narrow  valley  from  one  to  two  miles 
wide  all  round  the  Wealden  district  between  the  Lower  Green- 
sand  and  the  Chalk,  both  of  which  form  high  ground.  This 
valley  is  due  to  the  more  rapid  denudation  of  the  soft  clay, 
which  is  often  very  wet  and  full  of  springs,  since  it  throws  out 
the  water  from  the  Chalk  above.  The  Gault  weathers  into  a 
brown  or  nearly  white  soil :  the  latter  strongly  resembling  the 
Chalk  Marl  soils  above.  In  places  where  Gault  has  been 
naturally  mixed  with  sand  from  the  higher  ground  the  soil  is 
sometimes  good,  but  the  unmodified  clay  is  too  heavy  for  arable 
land  and  is  nearly  all  in  pasture.  The  total  area  is  not  large, 
and  in  the  west  where  the  outcrop  is  widest  it  is  to  a  considerable 
extent  under  oak  forest  (Alder  and  Alice  Holt,  west  of  Farnham 
in  Hampshire). 

In  the  Isle  of  Wight  the  Gault  clay  is  seen  chiefly  in  the 
cliffs  and  is  known  as  "blue  slipper,"  from  its  tendency  to 
produce  landslips,  bringing  down  masses  of  overlying  strata  and 
forming  the  well-known  "under- cliffs"  of  the  island. 


xiv]  THE   CRETACEOUS   SYSTEM  277 

The  Upper  Greensand,  the  sandy  facies  of  the  Gault,  hardly 
exists  in  Kent,  the  Gault  there  passing  gradually  into  the 
overlying  Chalk  Marl,  but  in  eastern  Surrey  it  begins  to  assume 
some  importance.  In  western  Surrey  and  on  the  borders  of 
Hampshire,  and  also  along  the  southern  outcrop  at  the  foot  of 
the  South  Downs  it  forms  a  strip  or  terrace  of  land  about  a 
mile  wide,  somewhat  elevated  above  the  low  Gault  valley.  In 
Surrey  and  Sussex  the  Upper  Greensand  generally  consists  of 
about  20  feet  of  white  or  greenish-grey  calcareous  sandstone, 
sometimes  almost  soft  and  incoherent,  but  in  other  places 
forming  a  good  building  stone,  often  known  as  Malmstone. 
It  thickens  towards  the  west  and  covers  a  considerable  area 
round  Farnham.  All  round  the  Wealden  area  the  Upper  Green- 
sand  forms  a  rather  heavy  soil,  dark  in  colour  when  wet,  but 
drying  very  white.  It  is  generally  fertile  and  in  some  parts 
carries  good  crops  of  hops,  wheat  and  mangolds.  There  is  little 
wood  and  no  waste  land.  Near  Reigate  a  thin  bed  of  hard  stone 
is  cemented  by  silica  instead  of  calcium  carbonate.  This 
resists  heat  well,  and  is  much  used  for  hearth-stones. 

The  Upper  Greensand  appears  again  in  the  Isle  of  Wight, 
and  in  Dorset  it  reaches  its  maximum  thickness  of  about  180 
feet,  thus  almost  wholly  replacing  the  Gault  clay.  Isolated 
patches  of  the  sandstone  and  chert  extend  even  to  the  Haldon 
Hills  beyond  Exeter.  In  Somerset  the  Blackdown  beds,  which 
are  of  about  the  same  age,  though  perhaps  representing  a  little 
of  the  Chalk  as  well,  are  of  rather  peculiar  character.  The 
Blackdown  Greensand  is  a  highly  siliceous  rock,  even  the  fossils 
consisting  of  silica.  It  yields  whetstones  or  scythe-stones  of 
good  quality.  In  the  south  of  Wiltshire  the  Upper  Greensand 
is  about  150  feet  thick,  at  Didcot  70  feet,  and  in  Buckingham- 
shire it  dies  out  altogether,  passing  laterally  into  the  clay  of  the 
Upper  Gault. 

In  Bedfordshire  and  Cambridgeshire  the  Gault  outcrops  over 
a  considerable  area,  always  forming  low  ground,  and  underlying 
the  eastern  part  of  the  Fens.  Consequently  it  is  largely  con- 
cealed by  a  thick  cover  of  river  gravels  and  alluvium.  Where 
exposed  at  the  surface  it  forms  a  heavy  cold  wet  soil,  of  much  the 
same  quality  as  the  Oxford  Clay.  It  always  needs  draining,  but 


278  THE   CRETACEOUS   SYSTEM  [CH. 

owing  to  the  low  levels  this  operation  is  often  difficult  to  carry 
out.  From  recent  observations  it  appears  that  certain  areas 
near  Cambridge  formerly  mapped  as  Gault  are  really  covered 
by  a  grey  clayey  or  marly  deposit.  Although  strongly  resem- 
bling Gault  in  appearance  this  clay  contains  fragments  of  flint 
derived  from  the  Chalk.  It  is  doubtless  largely  made  up  of 
material  derived  from  the  Gault  and  other  formations,  laid  down 
in  lakes  of  Pleistocene,  possibly  late  Glacial  age.  The  true 
Gault  soils  of  this  area  are  best  suited  for  permanent  pasture, 
although  there  is  a  good  deal  of  arable  land.  Where  gravels 
cover  the  Gault  a  good  deal  of  fruit  is  ^rown. 

The  Chalk.  From  the  agricultural  point  of  view  the  Chalk 
is  undoubtedly  one  of  the  most  important  of  British  rock- 
formations.  It  forms  a  large  proportion  of  the  surface  area  of 
England  in  the  south  and  east,  and  owing  to  a  variety  of  causes 
Chalk  soils  are  often  more  highly  farmed  than  their  actual 
agricultural  value  would  seem  to  warrant.  This  state  of  affairs 
is  brought  about  by  favourable  climate,  nearness  to  large 
markets  and  other  factors.  • 

The  Chalk  itself  is  a  remarkably  uniform  formation,  thus 
contrasting  strongly  with  the  strata  below  it.  Local  variations 
in  the  soil  are  due  to  superficial  deposits  of  various  kinds  masking 
the  character  of  the  Chalk.  Almost  everywhere  the  Chalk 
follows  directly  on  the  beds  of  the  Gault  or  Upper  Green  sand; 
only  in  Cambridgeshire  is  there  any  real  break  in  the  succession 
(see  p.  102).  Where  the  Upper  Greensand  is  not  developed  the 
top  of  the  Gault  becomes  more  and  more  calcareous  till  it  passes 
into  the  Chalk  Marl,  the  lowest  zone  of  the  Chalk.  In  the 
south  of  England  the  base  of  this  division  is  marked  by  the 
so-called  Chloritic  Marl,  a  layer  of  soft  calcareous  sandy 
clay  with  glauconite  and  phosphate  nodules.  Where  the 
Upper  Greensand  is  found  the  transition  is  naturally  more 
abrupt. 

The  Chalk  Marl  is  soft  and  forms  low  ground,  but  the  rest 
of  the  Chalk  nearly  always  stands  up  as  conspicuous  hills,  such 
as  the  North  and  South  Downs,  Salisbury  Plain,  the  Chiltern 
Hills  and  the  Wolds  of  Lincolnshire  and  Yorkshire.  The 
Chalk  hills  are  lowest  between  Hertfordshire  and  the  Wash; 


xiv]  THE   CRETACEOUS   SYSTEM  279 

this  is  perhaps  due  to  reduction  in  height  by  ice  during  the 
glacial  period. 

The  Chalk  is  generally  subdivided  into  three  series,  Lower, 
Middle  and  Upper.  Still  smaller  subdivisions  are  partly  litho- 
logical  and  partly  founded  on  fossil  zones.  However  the 
lithological  variations  are  but  slight,  being  for  the  most  part 
only  slight  differences  of  colour  and  hardness.  The  oldest 
classification  is  founded  on  the  relative  abundance  or  absence 
of  flints,  as  follows: 

Upper  Chalk  with  many  flints, 

Middle  Chalk  with  few  flints, 

Lower  Chalk  without  flints. 

This  classification  is  not  universally  applicable,  since  there 
are  many  local  variations  in  this  respect,  though  it  is  useful 
as  a  rough  generalization.  The  flints  derived  from  the  Chalk 
are  of  much  importance  in  all  the  succeeding  stages  of  the 
British  formations,  at  any  rate  in  the  south  and  east  of  England, 
since  they  form  the  source  of  the  material  of  a  very  large 
proportion  of  the  Tertiary  strata,  and  of  the  recent  superficial 
deposits.  In  all  of  these  flint  gravels  are  very  common.  For 
an  account  of  the  origin  of  flints  see  p.  99. 

In  the  south-east  of  England  the  Lower  Chalk  is  about  200 
feet  thick,  but  it  decreases  towards  the  west,  and  in  Devonshire 
it  is  represented  by  3  or  4  feet  only  of  gritty  pebbly  sandstone, 
evidently  laid  down  in  very  shallow  water.  In  the  south  of 
England  the  Middle  Chalk  also  averages  about  200  feet;  both 
divisions  also  become  thinner  towards  the  north.  Although 
the  highest  zones  of  the  continental  Chalk  are  not  found  in  this 
country,  nevertheless  the  Upper  Chalk  is  at  least  1300  feet  thick 
in  Hampshire,  and  about  1000  feet  at  Norwich. 

The  Chalk  may  be  described  in  general  terms  as  a  remarkably 
pure  limestone,  sometimes  containing  as  much  as  98  per  cent, 
of  carbonate  of  lime.  The  lower  part  however  is  often  much 
less  pure  than  this,  the  Chalk  Marl  in  particular  containing 
enough  clay  to  make  good  cement.  The  Chalk  varies  in  colour 
from  pure  white  to  various  shades  of  pale  grey  and  pale  yellow, 
or  even  pinkish.  The  rock  is  generally  much  divided  by 
irregular  joints,  and  varies  a  good  deal  in  hardness;  the  Chalk 


280  THE   CRETACEOUS   SYSTEM  [CH. 

of  Lincolnshire  and  Yorkshire  is  much  harder  than  that  south 
of  the  Wash,  while  the  Chalk  of  Antrim  is  the  hardest  of  all1. 

Perhaps  the  most  important  practical  variation  in  the  Chalk 
is  in  relation  to  its  phosphoric  acid  content.  In  some  places 
only  the  merest  trace  of  this  substance  is  present,  but  in  a  few 
localities  it  is  sufficiently  abundant  to  produce  a  marked  effect 
on  the  fertility  of  the  soil,  and  even  to  yield  substances  of  ma- 
nurial  value.  The  peculiar  phosphatic  deposit  of  the  Cambridge 
Greensand  has  already  been  described  (see  p.  102).  This 
however  is  quite  exceptional,  being  the  direct  result  of  local 
uplift  and  unconformity.  Under  ordinary  circumstances  the 
phosphate  occurs  either  disseminated  throughout  the  Chalk  or 
as  ill-defined  lumps  and  nodules  not  differing  much  in  appear- 
ance from  the  rest  of  the  rock.  In  the  well-known  phosphatic 
Chalk  at  Taplow  the  phosphate  is  due  to  the  presence  of  fish- 
remains  in  great  abundance.  A  greyish  brown  phosphatic  Chalk 
is  also  found  at  Ciply  in  Belgium.  (For  further  details  see  p.  103.) 

The  distribution  of  the  Chalk  can  be  most  conveniently 
described  by  taking  Salisbury  Plain  as  a  starting  point.  From 
this  centre  Chalk  hills  radiate  in  four  directions:  (1)  towards 
the  south-west  into  Dorset  and  the  east  of  Devonshire; 
(2)  slightly  south  of  east  into  Hampshire  and  Sussex,  ending  in 
the  South  Downs  at  Beachy  Head ;  (3)  nearly  due  east,  diverging 
from  the  last  and  running  through  the  North  Downs  to  Dover ; 
(4)  towards  the  north-east,  as  far  as  Suffolk — in  Norfolk  the 
strike  becomes  due  north,  in  Lincolnshire  and  south  Yorkshire 
north-west.  Finally  the  outcrop  of  the  Chalk  swings  round  to 
the  east,  ending  just  north  of  Flamborough  Head.  There  are 
also  small  isolated  patches  in  the  Isle  of  Wight  and  in  the  Isle 
of  Thanet.  Each  of  these  six  areas  possesses  its  own  agricultural 
characteristics,  and  must  be  separately  described. 

Perhaps  the  most  typical  of  all  Chalk  areas  is  the  South 
Downs ;  here  the  Chalk  is  quite  free  from  transported  material, 
unlike  many  tracts  further  north,  and  true  Chalk  soils  are 
found,  giving  rise  to  a  characteristic  landscape,  specially 

1  The  Chalk  of  Antrim  represents  only  the  Upper  Chalk  of  England.  The 
Irish  equivalents  of  the  Lower  and  Middle  Chalk  are  soft  glauconitic  sand- 
stones, the  Hibernian  Greensand. 


xiv]  THE   CRETACEOUS   SYSTEM  281 

remarkable  from  the  absence  of  running  streams.  A  few  rivers 
rise  in  the  Weald  and  cut  across  the  Downs,  but  with  this 
exception  all  the  valleys  are  dry  (see  also  p.  124).  The  open 
Downs  are  generally  covered  by  a  thin  layer  of  red  flinty  soil, 
a  true  residual  soil,  representing  the  insoluble  residue  of  the 
Chalk.  This  soil  is  deficient  in  lime,  so  much  so  as  to  require 
calcareous  manures.  In  places  where  there  has  been  less  solution 
the  soil  is  pale  grey  or  white  and  very  calcareous,  and  admirably 
adapted  for  sheep-farming,  this  being  the  staple  industry.  The 
higher  parts  are  generally  open  grassy  pasture,  with  arable  land 
in  the  hollows,  where  the  sheep  are  folded  for  fattening. 

On  the  North  Downs  a  very  large  proportion  of  the  Chalk 
is  covered  by  the  clay-with-flints,  which  seriously  alters  the 
character  of  the  soil  (see  p.  114).  Where  free  from  this  deposit 
the  Chalk  soils  are  of  much  the  same  kind  as  in  the  South  Downs, 
but  in  Surrey  and  western  Kent  the  character  of  the  farming  is 
largely  controlled  by  proximity  to  London,  from  whence  also 
large  supplies  of  manure  are  obtained.  Hence  arable  land  is 
more  abundant  than  pasture,  and  the  strip  of  country  along 
the  north  slope  of  the  North  Downs  is  very  highly  farmed, 
especially  for  potato-growing  and  dairying.  The  Chalk  area 
of  the  Isle  of  Thanet  is  almost  entirely  under  the  plough, 
without  trees  or  hedges. 

The  Down  type  of  scenery  is  also  to  be  seen  over  a  wide 
extent  of  country  in  Hampshire,  Wiltshire,  Dorsetshire  and 
Berkshire.  Originally  almost  or  quite  free  from  forest,  this  was 
one  of  the  first  districts  to  be  cultivated  regularly  on  a  large 
scale,  and  has  long  been  devoted  to  sheep-farming.  Taking 
Salisbury  Plain  as  an  example,  the  soils  on  the  Chalk  Marl  in 
the  lower  parts  of  the  valleys  are  generally  heavy,  and  often 
used  as  water-meadows.  The  slopes  of  the  hills  are  commonly 
arable  land,  with  open  sheep-walk  at  the  top.  The  arable  soils 
are  free-working  loams,  rather  heavy  below,  light  and  thin  in 
the  higher  parts,  and  often  very  flinty.  The  highest  arable 
soils  are  thin  (often  only  4  or  5  inches),  black  and  crowded  with 
small  flints,  with  pure  Chalk  immediately  below,  there  being  no 
real  subsoil.  The  open  land  is  all  grass,  with  the  characteristic 
short  sweet  herbage  of  the  Chalk.  The  farms  are  usually  laid 


282  THE   CRETACEOUS  SYSTEM  [CH.  xiv 

out  in  long  strips  running  down  the  hill  sides,  so  as  to  include 
a  fair  proportion  of  all  kinds  of  land. 

In  Dorset  the  Chalk  soil  is  usually  a  yellowish  flinty  loam, 
the  lower  slopes  being  arable  and  the  uplands  grass.  On  the 
high  ground  about  Winchester  the  soil  is  of  the  common  down- 
land  type,  grey  or  brown  in  colour,  light  and  loamy,  often 
forming  a  mere  skin  of  turf  over  the  Chalk;  it  is  chiefly  in 
pasture,  with  some  woods.  On  the  lower  slopes  the  soil  is 
generally  a  red-brown  loam,  often  rather  heavy,  but  thin. 
Near  Basingstoke  the  soils  vary  a  good  deal,  being  chiefly  loams 
or  marls,  often  stony  and  thin;  the  Chalk  here  carries  many 
large  patches  of  clay-with-flints  which  yield  a  much  heavier 
reddish  brown  soil.  The  Chalk  Downs  of  Berkshire  are  mostly 
under  natural  herbage,  while  the  beech  woods  of  the  Chiltern 
Hills  are  a  well-known  and  characteristic  feature.  In  Hertford- 
shire and  Cambridgeshire  the  Chalk  is  largely  covered  by  glacial 
gravels  and  boulder-clay.  Where  free  from  these  deposits  the 
Chalk  yields  a  thin  white  or  grey  loamy  soil,  nearly  all  under 
the  plough,  and  carrying  large  areas  of  turnips  and  of  sainfoin. 
Grass  land  is  scarce,  although  there  are  a  few  areas  such  as 
Newmarket  Heath,  covered  with  the  natural  short  turf  of  the 
Down  type.  In  Suffolk  and  Norfolk  the  Chalk  is  generally 
buried  under  a  thick  cover  of  glacial  and  other  superficial 
deposits ;  true  Chalk  soils  are  rarely  to  be  found  in  these 
counties.  On  the  Lincoln  Wolds  the  soils  are  usually  very 
thin,  with  little  or  no  subsoil ;  there  are  no  open  downs  and  little 
permanent  pasture  of  any  kind.  The  Yorkshire  Wolds  again 
form  a  typical  sheep,  turnip  and  barley  district.  The  soil  is  a 
light  loam,  flinty  in  places,  but  very  free  from  superficial 
deposits;  the  land  is  nearly  all  arable,  and  both  fields  and 
farms  run  large.  In  the  Wold  valley  which  cuts  through  the 
centre  of  the  district  from  Malton  towards  Driffield  the  soils  are 
deeper  and  richer  and  well  suited  to  wheat.  Much  of  the  higher 
parts  of  the  Wolds  were  brought  under  the  plough  in  com- 
paratively recent  times,  having  formerly  been  largely  rabbit 
warrens  and  open  sheep  walks.  Chalk  also  underlies  the  whole 
of  the  plain  of  Holderness,  but  is  too  deeply  buried  under  drift 
to  have  any  influence  on  the  character  of  the  soils. 


CHAPTER   XV 

THE   TERTIARY   SYSTEMS 

The  Tertiary  rocks  are  by  most  authorities  divided  into  four 
separate  systems,  Eocene,  Oligocene,  Miocene  and  Pliocene. 
This  classification  leaves  somewhat  doubtful  the  position  of 
the  deposits  later  than  the  Pliocene,  which  are  commonly 
divided  into  Pleistocene  and  Recent;  by  some  writers  these 
are  regarded  as  included  in  the  Tertiary,  while  others  introduce 
another  term,  Quaternary.  Without  entering  into  a  discussion 
of  the  merits  of  these  classifications  it  will  be  sufficient  to  state 
here  that  the  strata  from  Eocene  to  Pliocene  inclusive  are 
included  in  this  chapter,  while  the  Pleistocene  and  Recent 
deposits  are  treated  in  a  separate  chapter.  This  course  is 
sufficiently  justified  by  the  great  importance  from  an  agricul- 
tural standpoint  of  the  latter  groups. 

Between  the  deposition  of  the  highest  beds  of  the  Chalk 
seen  in  Britain  and  the  lowest  beds  of  the  Eocene  a  long  period 
of  time  must  have  elapsed  and  there  was  also  a  striking  change 
in  the  physical  conditions.  The  fairly  deep  and  clear  sea  of 
the  Chalk  gave  way  to  shallow  water  and  estuarine  conditions ; 
the  greater  part  of  the  area  was  elevated  into  land  and  near  its 
shores  beds  of  gravel,  sand  and  mud  were  laid  down ;  in  the 
north-east  of  Ireland  and  in  the  west  of  Scotland  there  was 
great  volcanic  activity  in  the  earlier  part  of  the  period. 

Throughout  the  greater  part  of  Tertiary  time  there  were 
important  earth-movements  in  nearly  all  quarters  of  the  world, 
leading  to  the  formation  of  great  mountain- chains ;  most  of  the 
important  mountain-ranges  of  the  world  date  from  this  time. 
The  effect  of  these  movements  was  felt  in  nearly  all  parts  of  the 
British  Isles,  though  not  so  strongly  as  on  the  continent ;  the 
Jurassic  and  Cretaceous  rocks  were  tilted  and  in  parts  of  the 


284  THE   TERTIARY  SYSTEMS  [CH. 

south  of  England  strongly  folded  and  most  of  the  existing 
structure  and  relief  of  the  British  Isles  date  'from  Miocene 
times,  when,  as  it  appears,  our  present  river-systems  were 
chiefly  initiated. 

Between  the  Cretaceous  and  the  Tertiary  there  is  also,  in 
Britain  at  any  rate,  a  great  palaeontological  break.  Very  few, 
if  any,  of  the  Cretaceous  species  of  fossils  are  found  in  the 
Tertiary,  and  the  fauna  of  the  latter  has  a  distinctly  modern 
appearance.  The  existing  species  of  shells,  for  example,  begin 
to  appear  in  the  Eocene,  and  in  the  Pliocene  nearly  all  the 
invertebrates  belong  to  still  living  species.  In  the  Tertiary 
strata  also  we  find  the  ancestors  of  the  present  vertebrates, 
including  those  of  our  domestic  animals;  this  subject  is  dealt 
with  in  Chapter  xvn. 

It  should  be  stated  however  that  the  stratigraphical  break 
between  Cretaceous  and  Tertiary  is  not  everywhere  so  apparent 
as  in  Britain.  In  the  Mediterranean  region  and  in  the  United 
States,  out  of  many  other  examples,  there  are  strata  clearly 
intermediate  between  the  two ;  in  Britain  both  the  Cretaceous 
and  the  Eocene  are  somewhat  exceptional  in  character,  belonging 
to  inland  sea  and  shallow  water  facies,  whereas,  as  a  rule  it  is 
only  in  the  open  sea  that  perfect  continuity  is  found,  as  occurs 
in  the  deposits  of  this  age  in  the  south  of  Europe  and  northern 
Africa. 

I.     THE  PALAEOGENE  SYSTEM.     EOCENE  AND  OLIGOCENE 

SERIES 

Of  the  subdivisions  of  the  Tertiary  the  Eocene  alone  covers 
an  extensive  area  in  the  British  Isles.  There  are  now  two 
isolated  patches  of  considerable  size,  known  as  the  London 
basin  and  the  Hampshire  basin  respectively.  The  London 
basin  forms  a  triangular  area  extending  from  the  eastern  border 
of  Wiltshire  to  the  North  Sea;  on  the  north  it  extends  as  far 
as  Ipswich  while  its  southern  limit  is  at  Deal,  in  Kent.  The 
subdivisions  generally  recognized  are  as  follows: 

Bagshot  Sands, 

London  Clay, 

Lower  London  Tertiaries. 


xv]  THE   TERTIARY  SYSTEMS  285 

The  lowest  division  is  thickest  in  the  south  and  thins  to  the 
north.     It  is  usually  further  subdivided  thus: 
Blackheath  and  Oldhaven  beds, 
Woolwich  and  Reading  beds, 
Thanet  Sands. 

At  the  base  of  the  Thanet  Sands  there  is  generally  a  peculiar 
bed  of  green-coated  flints ;  it  is  supposed  that  after  the  sands 
were  deposited  solution  of  the  Chalk  went  on  and  these  flints 
were  left  behind.  For  this  reason  also  the  base  of  the  Thanet 
Sands  is  very  irregular  and  they  have  often  fallen  into  deep 
pipes  formed  by  solution  in  the  Chalk.  The  upper  part  of  the 
series  consists  of  sands  and  flint-gravels  with  exclusively 
marine  shells. 

The  Woolwich  and  Reading  beds  consist  partly  of  sands 
and  partly  of  clays,  the  latter  being  dominant  in  the  west  and 
north.  The  Blackheath  and  Oldhaven  beds,  composed  of 
sands  and  gravels  are  thin  and  quite  unimportant.  The  total 
thickness  of  the  whole  of  the  Lower  London  Tertiaries  is 
generally  less  than  100  feet ;  their  chief  importance  lies  in  the 
fact  that  in  many  places,  especially  to  the  north  of  the  Thames 
valley,  they  occur  as  small  outliers  resting  on  the  Chalk,  and  they 
have  undoubtedly  supplied  material,  mainly  flints,  to  many  of 
the  superficial  deposits  mentioned  in  the  next  chapter  (see  also 
Chapter  v).  In  some  localities  certain  beds  have  been  cemented 
by  silica  to  form  hard  and  massive  grits  and  conglomerates, 
including  the  Sarsen  stones  of  the  Wiltshire  downs  and  the 
Hertfordshire  Pudding  Stone.  The  distribution  of  the  Sarsen 
stones,  as  for  example  near  Marlborough  and  on  Salisbury 
plain,  shows  that  the  Eocene  beds  once  extended  much  further 
than  they  do  now,  having  been  extensively  removed  by  denu- 
dation from  the  elevated  ground.  In  the  Hampshire  basin  only 
the  Reading  beds  are  present,  so  that  deposition  must  have 
begun  somewhat  later  in  this  area,  but  as  shown  later  the  two 
now  separated  areas  were  once  continuous. 

The  London  Clay  covers  by  far  the  greater  part  of  the 
London  basin  and  is  also  found  in  the  Hampshire  basin.  At 
its  thickest  it  comprises  about  500  feet  of  stiff  clay,  dark  blue 
in  colour  when  obtained  from  deep  borings  and  wells,  but 


286  THE   TERTIARY   SYSTEMS  [CH. 

weathering  brown  near  the  surface.  It  is  of  estuarine  origin 
and  contains  a  mixture  of  marine  shells  and  land  plants ;  the 
latter  especially  are  of  an  almost  tropical  character,  including 
palms  and  magnolias.  Towards  the  west  the  clay  becomes 
more  sandy,  as  also  in  Sussex  and  Hampshire.  It  was  clearly 
deposited  at  and  near  the  mouth  of  a  large  river  that  flowed 
from  an  extensive  land  area  in  the  west. 

After  the  deposition  of  the  London  Clay  the  conditions  in 
the  northern  and  southern  parts  of  the  area  of  deposition  show 
a  considerable  difference;  in  the  Thames  valley  the  upper 
Eocene  strata  consist  of  a  considerable  thickness  of  sands,  the 
Bagshot  Sands,  which  form  the  soil  of  the  well-known  heathy 
barren  country  about  Aldershot,  Bagshot,  Woking  and  Ascot. 
There  are  also  several  small  outliers  capping  the  hills  at  Harrow, 
Hampstead  and  Highgate  in  the  northern  suburbs  of  London, 
together  with  some  patches  in  Essex  (Epping  Forest).  The 
maximum  thickness  is  about  150  feet  and  fossils  are  scarce.  In 
Hampshire  and  the  Isle  of  Wight  on  the  other  hand  the  upper 
Eocene  strata  consist  of  clays  and  sands  with  occasional  pebble 
beds,  containing  as  a  rule  abundant  marine  fossils  and  some 
fresh- water  beds  with  plants.  Some  parts  of  this  series  strongly 
resemble  the  Bagshot  Sands,  and  there  is  no  doubt  of  their 
general  equivalence.  The  fossils,  both  shells  and  plants, 
indicate  a  warm  sub-tropical  climate. 

In  the  Isle  of  Wight  and  in  the  New  Forest  the  Eocene 
strata  are  succeeded  conformably  by  the  Oligocene  series,  a 
variable  succession  of  sands,  clays  and  limestones,  mainly  of 
fresh-water  origin,  and  containing  abundant  land  and  fresh- 
water shells,  with  some  shallow-water  marine  bands.  The.  total 
thickness  is  600  or  700  feet,  but  the  area  covered  is  small  and 
the  whole  series  is  of  little  agricultural  importance,  the  New 
Forest  being  largely  unenclosed  and  covered  with  heather  and 
pine  woods.  Oligocene  strata  are  not  found  in  any  other  part 
of  the  British  Isles. 

As  before  stated  there  was  during  Eocene  and  probably  also 
Oligocene  times  great  volcanic  activity  in  the  north-western 
part  of  Britain,  in  the  north-east  of  Ireland  and  the  west  of 
Scotland;  this  was  an  accompaniment  of  the  great  crust 


xv]  THE   TERTIARY  SYSTEMS  287 

disturbances  of  that  period,  which  in  Britain  culminated  in 
the  Miocene.  The  volcanic  eruptions  belonged  to  a  special  type 
known  as  fissure-eruptions;  there  were  no  volcanic  cones,  but 
the  basaltic  lava  welled  out  from  great  cracks  in  the  crust,  and 
accumulated  to  a  depth  of  two  or  three  thousand  feet  in 
successive  flows,  with  occasional  sedimentary  deposits  and  old 
surface  soils  between  the  flows,  testifying  to  a  considerable 
lapse  of  time.  At  a  later  stage  great  laccoliths  of  granite  and 
gabbro  were  intruded  into  the  lavas,  as  well  as  a  series  of  dolerite 
sills  and  dykes.  Though  originally  of  much  wider  extent  than 
now,  the  basalt  plateaux  cover  considerable  areas  in  Antrim, 
Mull  and  Skye,  while  intrusions  of  this  age  are  also  found  in  the 
Mourne  Mountains  and  in  Arran,  Rum,  Eigg  and  other  islands 
of  the  Inner  Hebrides.  Some  very  large  dykes  in  southern 
Scotland  and  northern  England  also  belong  to  this  period. 


II.     THE  MIOCENE 

The  very  limited  distribution  of  Oligocene  strata  in  Britain 
shows  that  the  greater  part  of  the  area  was  already  land  at 
this  time,  and  during  the  succeeding  period  this  state  of  things 
continued  in  an  even  more  marked  degree ;  consequently  Miocene 
strata  are  entirely  absent.  This  period  was  almost  everywhere 
in  western  Europe  a  time  of  crust-disturbance,  denudation  and 
continental  conditions.  The  tilting  and  folding  of  the  strata 
are  very  conspicuous  in  the  south  of  England.  From  this  time 
dates  the  anticlinal  uplift  of  the  Weald,  which  in  its  westward 
continuation  into  Hampshire  and  Wiltshire  brought  up  to  the 
surface  a  broad  band  of  Chalk  and  separated  the  Eocene  strata 
into  two  distinct  synclinal  basins,  one  in  the  Thames  valley 
and  the  other  in  Hampshire  and.  the  northern  half  of  the  Isle  of 
Wight.  Another  fold  parallel  to  the  Wealden  axis  affected  the 
Cretaceous  rocks  of  this  island  and  of  southern  Dorset.  It 
forms  a  sharp  monocline  and  the  Chalk  and  lower  Eocene  strata 
are  now  almost  vertical,  as  can  be  well  seen  at  the  western  end 
of  the  Isle  of  Wight.  The  Miocene  uplift  also  gave  rise  to  the 
dominant  south-easterly  and  easterly  dip  of  the  Mesozoic  rocks 


288  THE   TERTIARY  SYSTEMS  [CH. 

of  England  as  far  north  as  Yorkshire;  the  structure  of  the 
country  then  became  essentially  as  it  now  exists.  The  Wealden 
axis  underwent  a  further  slight  uplift  somewhat  later,  but  apart 
from  this  there  has  been  little  differential  movement,  although 
some  variation  has  taken  place  in  the  relative  levels  of  land 
and  sea. 

The  deposits  generally  regarded  as  typical  of  the  Miocene 
of  western  Europe  are  the  Faluns  of  Touraine.  These  are 
shelly  and  marly  sands,  formerly  used  to  a  considerable  extent 
for  spreading  on  the  land  as  a  fertilizer.  They  are  only  of 
interest  from  their  strong  resemblance  to  the  Pliocene  beds  of 
East  Anglia,  to  be  described  in  the  next  section. 


III.     THE  PLIOCENE 

At  the  close  of  Miocene  time  the  sea  again  encroached  on 
the  land  over  a  part  of  southern  and  eastern  England,  from 
Sussex  to  Norfolk,  and  also  in  Cornwall  and  South  Wales. 
South  of  the  Thames  deposits  of  this  age  are  very  scanty,  and 
another  uplift  of  five  or  six  hundred  feet  carried  some  patches  of 
early  Pliocene  gravels  and  sands  to  the  top  of  the  North  Downs 
in  Kent,  where  some  small  remains  of  them  are  still  preserved 
in  pipes  in  the  Chalk.  In  Cornwall,  South  Wales  and  Wexford 
also  a  few  patches  of  gravel  still  remain  on  the  surface  of  the 
uplifted  plain  of  marine  denudation. 

It  is  only  in  Essex,  Suffolk  and  Norfolk  that  Pliocene  strata 
cover  any  considerable  area  and  even  these  are  for  the  most 
part  masked  by  glacial  sands  and  gravels,  so  that  their  import- 
ance as  subsoils  is  slight.  The  Pliocene  strata  of  East  Anglia 
consist  in  the  main  of  sands  and  gravels  often  very  rich  in  shells, 
whole  or  broken,  and  strongly  resembling  the  Miocene  Faluns. 
To  these  deposits  the  local  agricultural  term  "  Crag"  is  commonly 
applied.  The  total  thickness  may  be  a  little  over  300  feet,  but 
the  whole  series  is  never  developed  at  any  one  place;  on  the 
contrary  the  beds  are  shown  to  be  successively  newer  when 
followed  from  south  to  north,  from  Walton-on-the-Naze  to 
Cromer. 


xv]  THE   TERTIARY   SYSTEMS  289 

The  whole  succession  may  be  generalized  as  follows : 

Cromer  Forest  beds, 

Weybourn  Crag, 

Chillesford  beds, 

Norwich  Crag, 

Red  Crag, 

Coralline  Crag. 

The  Coralline  Crag,  a  yellow  shelly  sand,  is  only  known  as 
a  small  patch  near  Aldeburgh.  The  Red  Crag  extends  over  an 
area  of  some  300  square  miles,  but  is  rarely  seen  at  the  surface 
owing  to  a  covering  of  glacial  deposits.  It  consists  of  reddish 
ferruginous  sands  and  gravels  with  very  abundant  shells,  often 
much  false-bedded,  and  clearly  formed  as  sand-banks  along  the 
coast  line  of  a  sea  that  was  retreating  to  the  north.  The 
Norwich  Crag  spreads  over  an  area  of  some  hundreds  of  square 
miles  in  eastern  Suffolk  and  Norfolk;  it  consists  of  sands, 
clays  and  gravels,  paler  in  colour  than  the  Red  Crag.  A  boring 
at  Lowestoft  passed  through  180  feet  of  it.  The  Chillesford 
beds  are  of  somewhat  different  character,  being  river  deposits, 
and  it  is  believed  that  they  were  formed  in  a  northward  con- 
tinuation of  the  Rhine;  they  cover  only  a  very  small  area. 
The  Weybourn  Crag  and  the  Cromer  Forest  beds  are  only  seen 
on  the  coast.  They  are  also  river  deposits  and  the  Forest  beds 
contain  the  stumps  of  many  drifted  tre^e,  which  once  formed 
"snags"  in  the  river. 

When  the  shells  of  the  Crags  are  examined  in  detail  certain 
very  important  conclusions  can  be  drawn,  throwing  much  light 
on  the  climatic  and  geographical  conditions  that  then  prevailed. 
The  great  majority  of  the  shells  belong  to  still  existing  species, 
the  proportion  of  such  increasing  upwards.  Furthermore  in 
the  earlier  Pliocene  strata  there  are  many  shells  of  species  now 
found  in  the  Mediterranean  region.  As  we  rise  in  the  succession 
the  number  of  these  diminishes  and  they  are  gradually  replaced 
by  more  and  more  northern  forms.  At  the  end  of  the  period 
it  is  clear  that  the  climate  was  thoroughly  arctic,  foreshadowing 
the  approach  of  the  glacial  period  that  immediately  succeeded. 
The  connexion  with  the  Mediterranean  region  was  apparently 
lost,  being  replaced  by  free  communication  with  the  Arctic 
R.  A.G.  19 


290  THE   TERTIARY  SYSTEMS  [CH.  xv 

Ocean.  This  change  of  geographical  conditions  undoubtedly 
assisted  the  advent  of  the  northern  ice  in  this  country.  In 
Norfolk  especially  the  Pliocene  deposits  are  succeeded  by  the 
Pleistocene  without  visible  break,  and  it  is  not  altogether  easy 
to  determine  where  the  line  of  demarcation  should  be  drawn. 
In  the  few  places  where  the  Crags  form  the  actual  surface, 
free  from  glacial  deposits,  they  yield  a  light  type  of  soil,  usually 
of  very  poor  quality. 


CHAPTER   XVI 

THE  PLEISTOCENE  AND  RECENT  FORMATIONS 

0  wing  to  the  absence  of  the  rest  of  the  Tertiary  strata  over 
the  greater  part  of  this  country,  the  Pleistocene  and  Recent 
formations  usually  rest  with  a  decided  unconformity  on  older 
rocks ;  as  before  mentioned  it  is  only  in  Norfolk  that  there  is 
any  real  transition.  This  subdivision  includes  all  deposits 
formed  between  the  end  of  the  Pliocene  and  the  present  day. 
It  is  usually  divided  into  two  stages,  Glacial  and  post-Glacial, 
but  this  is  unsatisfactory  since  the  Glacial  period  came  to  an 
end  in  different  places  at  different  times,  and  in  the  higher 
mountains  of  Europe  and  within  the  Arctic  circle  it  is  still 
going  on.  Hence  the  terms  glacial  and  post-glacial  will  here  be 
used  only  in  a  genetic  sense,  not  chronologically,  it  being  under- 
stood that  the  later  glacial  deposits  of  one  area,  e.g.  Switzerland 
or  Norway,  are  equivalent  in  time  to  the  post-glacial  deposits 
of  Great  Britain  or  of  northern  Germany. 

The  deposits  of  this  age  comprise  what  is  known  collectively 
to  the  Geological  Survey  as  drift.  An  older  and  still  more 
inappropriate  name,  diluvium,  still  lingers  on  the  continent, 
though  long  abandoned  in  Britain.  It  is  the  indication  of  the 
drift  deposits  in  this  sense  that  constitutes  the  difference 
between  the  "solid"  and  "drift"  editions  of  the  maps  of  the 
Geological  Survey.  The  only  exception  is  that  certain  great 
spreads  of  alluvium  and  peat  are  inserted  in  the  solid  maps, 
owing  to  the  absence  of  e±posures  and  the  impossibility  of 
ascertaining  what  lies  beneath  them. 

The  general  characters  of  the  glacial  deposits  have  already 
been  described  in  the  chapter  on  superficial  deposits  (Chapter  v). 

19—2 


292  THE   PLEISTOCENE   AND  [CH. 

It  remains  here  to  give  some  account  of  their  distribution  in 
the  British  Isles. 

It  has  already  been  stated  that  much  difference  of  opinion 
still  exists  as  to  the  exact  manner  in  which  the  lowland  glacial 
deposits  of  Britain  were  formed.  It  is  admitted  by  all  that 
valley  glaciers  of  the  Alpine  type  existed  in  the  mountains  of 
Scotland,  northern  England,  Wales  and  Ireland.  The  main 
controversy  is  concerned  with  the  genesis  of  the  boulder-clays, 
gravels  and  sands  of  the  lower-lying  districts  and  especially 
with  those  of  the  Midlands  and  eastern  England.  The  most 
striking  feature  is  the  existence  in  these  of  innumerable  boulders 
of  far-travelled  rocks,  that  must  have  been  brought  from 
northern  England,  Scotland  and  Norway.  The  last  are  of 
special  interest,  since  their  origin  is  so  well-established.  Owing 
to  the  distinctive  character  of  many  of  the  Norwegian  rocks 
there  can  be  no  possible  doubt  as  to  their  source.  The  unmis- 
takable igneous  rocks  of  the  Christiania  district,  and  especially 
one  variety  known  as  "rhomb-porphyry,"  have  been  found  all 
over  eastern  England,  in  Yorkshire,  Lincolnshire,  Norfolk, 
Cambridgeshire,  as  far  south  as  the  northern  suburbs  of  London, 
and  as  far  west  as  Bedford.  Hence  it  is  clear  that  Norwegian 
ice  advanced  to  the  coast  and  penetrated  far  up  the  valleys  of 
the  Midland  rivers.  Associated  with  these  boulders  are  also 
many  from  Scotland  and  northern  England,  including  abundant 
blocks  of  Carboniferous  Limestone  and  Millstone  Grit,  as  well 
as  igneous  rocks  of  many  kinds.  On  the  western  side  rocks 
from  south-western  Scotland  and  the  Lake  District  are  abundant 
in  Cheshire,  Staffordshire  and  Shropshire,  while  Welsh  boulders 
extend  as  far  east  as  Birmingham.  Another  striking  fact  is  the 
transport  of  boulders  of  granite  and  other  rocks  from  the  Lake 
District  over  the  Pennine  chain  into  east  Yorkshire,  both  down 
the  Tees  to  the  coast  and  down  the  Vale  of  York.  Again, 
northern  rocks  are  found  on  the  north  coast  of  Wales  and 
Scotch  rocks  in  Ireland;  hence  it  is  clear  that  the  Irish  Sea 
also  was  filled  with  ice  coming  from  the  north,  which  travelled 
over  the  plains  of  south  Lancashire  and  Cheshire,  against  the 
natural  drainage  of  the  country.  The  valleys  of  central  and 
south  Wales  possessed  their  own  glacier  systems,  which  followed 


xvi]  RECENT   FORMATIONS  293 

in  the  main  the  natural  drainage  lines,  with  occasional  diver- 
gences. More  difficult  to  comprehend  is  the  state  of  affairs  in 
the  Midland  counties;  even  yet  it  has  not  been  decided  with 
any  certainty  whether  glaciers  existed  in  the  hills  of  south 
Yorkshire  and  Derbyshire.  However  certain  anomalies  in 
boulder-distribution  are  most  easily  explained  on  the  hypothesis 
of  a  late  development  of  glaciers  in  this  region  after  the  partial 
withdrawal  of  the  sea-ice.  It  appears  that  at  a  late  stage  there 
was  a  transport  of  boulders  across  Norfolk  and  Suffolk  from 
north-west  to  south-east,  and  a  similar  effect  can  also  be  traced 
in  Rutland  and  Lincolnshire.  This  was  probably  due  to  a 
Pennine  glacier.  Boulder-clays  and  other  glacial  deposits  are 
abundant  in  Leicestershire,  Northamptonshire,  Warwickshire, 
and  almost  as  far  south  as  Oxford.  These  appear  to  be  of 
complex  nature,  due  to  ice-streams  coming  from  different 
directions.  Over  a  large  part  of  this  area  Chalk  and  flints  are 
common  in  the  drifts  and  the  influence  of  the  North  Sea  ice 
seems  to  have  extended  far  to  the  west,  though  its  exact  limits 
are  as  yet  unknown. 

In  Scotland  the  relations  are  somewhat  more  simple.  In 
general  terms,  glaciers  moved  down  all  the  great  valleys  towards 
the  sea,  overrunning  the  lower  lands  and  leaving  thick  deposits. 
The  Isle  of  Skye  had  a  small  independent  ice-cap,  with  its  centre 
in  the  Cuillin  and  Red  Hills.  The  behaviour  of  the  eastward 
flowing  glaciers  of  the  mainland,  when  they  encountered  the 
North  Sea  ice,  is  still  a  matter  of  uncertainty.  The  ice  from  the 
Moray  Firth  seems  to  have  been  driven  north-westwards  over 
Caithness,  but  south  of  Aberdeen  the  evidence  is  conflicting. 
Rocks  from  the  valley  of  the  Forth,  near  Edinburgh,  have  been 
identified  in  considerable  numbers  in  Norfolk  and  Cambridge- 
shire and  probably  some  from  Forfar  and  Aberdeen.  However 
some  Scotch  geologists  maintain  that  the  ice  from  the  Forth 
and  Tay  chiefly  went  northwards. 

There  was  a  great  development  of  glaciation  in  Ireland,  the 
structure  of  this  island  being  very  favourable  to  the  accumu- 
lation of  an  ice-sheet,  since  most  of  the  mountains  are  near  the 
coast,  with  a  low  plain  in  the  middle.  The  conditions  must 
have  been  very  similar  to  those  now  prevailing  in  Greenland. 


294  THE   PLEISTOCENE   AND  [CH. 

The  general  movement  was  radially  outwards  in  all  directions 
except  in  the  north-east,  where  the  local  ice  was  overpowered 
and  driven  back  by  that  coming  from  Scotland. 

The  glacial  deposits  consist  of  clays,  sands  and  gravels ; 
these  vary  in  their  constitution  and  character  according  to  the 
source  whence  they  are  derived.  The  boulder-clays  show  wide 
variations  both  in  the  nature  of  the  included  blocks  and  in  the 
finer  matrix.  In  most  cases  it  is  clear  that  the  greater  part 
of  the  material  is  of  local  origin,  having  been  derived  from  the 
region  passed  over  by  the  ice  just  before  reaching  the  spot  where 
the  examination  is  made.  Thus  in  eastern  England  the  con- 
stituents of  the  boulder-clay  were  evidently  derived  chiefly 
from  the  north-east  or  north;  in  the  western  Midlands  chiefly 
from  the  north-west  and  so  on.  In  the  glacial  deposits  of  the 
mountainous  regions  the  clay  as  well  as  the  stones  have  come 
from  the  higher  parts  of  the  country.  Since  argillaceous 
material  is  often  somewhat  scarce  in  mountains,  the  boulder- 
clays  of  such  regions  are  often  very  stony  indeed,  partaking 
more  of  the  nature  of  coarse  angular  gravels ;  in  fact  they  are 
just  like  the  moraines  of  existing  glaciers.  A  somewhat  more 
difficult  problem  is  the  origin  of  the  red  and  purple  stony  clays 
of  Lincolnshire  and  Yorkshire.  It  is  probable  that  they  are 
largely  composed  of  Triassic  material  derived  from  the  floor  of 
the  North  Sea.  The  Triassic  strata  reach  the  coast  in  north 
Yorkshire  and  we  have  no  means  of  ascertaining  how  far  they 
extend  to  the  eastward  of  the  present  coast-line.  Fragments  of 
Lias  and  other  Jurassic  rocks  are  very  numerous  in  some  of  the 
eastern  drifts,  and  the  very  abundant  Chalk  was  derived  from 
the  Lincolnshire  Wolds  and  from  the  neighbourhood  of  the 
Wash.  There  can  be  no  doubt  that  the  Chalk  hills  of  south 
Lincolnshire  and  of  Norfolk  were  much  reduced  in  height  by 
the  passage  of  the  ice  and  before  the  glacial  period  the  Wolds  of 
Lincolnshire  probably  extended  further  south  than  now.  The 
pebbles  of  Chalk  so  common  in  the  clays  and  gravels  of  Norfolk 
and  Cambridgeshire  are  much  harder  than  the  local  Chalk, 
resembling  that  of  Lincolnshire  and  Yorkshire  in  this  respect. 
In  East  Anglia  also  are  many  tabular  grey  flints  of  Lincolnshire 
type,  quite  unlike  the  local  black  flints.  In  the  drifts  of  Cheshire 


xvi]  KECENT  FORMATIONS  295 

and  Lancashire  are  many  well-rounded  sand-grains  derived  from 
the  Trias  sandstones. 

On  the  whole  organic  remains  are  not  very  abundant  in  the 
drifts ;  marine  shells,  whole  or  broken,  are  found  in  some  gravels 
in  Yorkshire,  Lincolnshire  and  elsewhere,  near  the  eastern  coast. 
Of  more  interest  is  the  occurrence  of  marine  shells  in  some 
places  in  North  Wales  and  Cheshire  at  considerable  heights. 
The  most  famous  of  these  occurrences  is  at  Moel  Tryfan  in 
Caernarvonshire,  where  some  sixty  species  of  shells  have  been 
found  on  a  mountain  top,  nearly  1400  feet  above  sea-level. 
These  have  been  brought  forward  as  proofs  both  of  the  land-ice 
and  of  the  submergence  theories.  Some  teeth  of  elephants  and 
bones  of  other  animals  found  in  eastern  England  have  been 
derived  from  Pliocene  deposits. 

With  the  evidence  at  present  at  our  disposal  it  is  impossible 
to  decide  the  vexed  question  whether  the  whole  glacial  history 
of  the  British  Isles  was  comprised  in  one  single  advance  and 
retreat  of  the  ice-sheet,  as  was  at  least  tacitly  assumed  by  many 
writers  till  within  very  recent  years,  or  whether  there  have  been 
several  glaciations  separated  by  interglacial  periods  of  mild 
climate.  In  Switzerland  it  has  been  shown  that  there  were 
four  separate  advances  with  intervening  periods  when  the  climate 
was  milder  than  at  present.  In  Norway  also  two  or  even  three 
such  periods  are  demonstrated.  It  is  to  say  the  least  very 
improbable  that  Britain  wholly  escaped  these  fluctuations,  but 
the  occurrence  of  warm  interglacial  periods  is  not  yet  con- 
clusively proved.  At  any  rate  it  is  well  known  that  in  some 
parts  of  England  there  are  two  or  even  three  boulder-clays  in 
vertical  succession,  derived  from  different  sources,  besides  a 
variety  of  sands  and  gravels,  as  in  the  east  of  England,  from 
Yorkshire  to  Suffolk.  The  clearest  evidence  of  climatic  fluctu- 
ations is  seen  in  the  peat-mosses  of  Scotland  (see  p.  119),  but 
it  has  not  yet  been  found  possible  to  correlate  these  with  the 
glacial  deposits  of  England.  We  do  not  yet  know  to  what 
extent  man  was  contemporaneous  with  the  glacial  period  in 
Britain ;  a  completely  glaciated  country  is  obviously  unfit  for 
colonization,  but  during  intervening  periods  of  mild  climate,  if 
such  occurred,  the  country  was  probably  inhabited.  There 


296  THE   PLEISTOCENE   AND  [CH. 

seems  no  doubt  at  any  rate  that  man  lived  in  Europe  during 
the  earlier  glaciations  of  the  Alps,  and  that  Great  Britain  was 
then  joined  to  the  continent,  so  that  migration  might  have 
occurred. 

The  southern  limit  of  true  glacial  deposits  in  England  coin- 
cides approximately  with  a  line  drawn  from  the  mouth  of  the 
Thames  to  the  Bristol  Channel,  with  a  bend  to  the  north  in  the 
neighbourhood  of  Oxford.  South  of  the  Thames  large  areas  of 
the  country  are  quite  free  from  drift,  though  large  spreads  of 
gravel  and  alluvium  are  often  found.  Some  of  these  must  be 
equivalent  in  age  to  the  glacial  drifts  of  the  north.  One  of  the 
most  important  types  is  the  Coombe  Rock  or  Head,  already 
described  (see  p.  124).  The  gravels  of  this  region  are  also 
described  in  the  chapter  on  superficial  deposits  (p.  125).  North 
of  the  Thames  almost  the  whole  country  is  masked  by  a  more 
or  less  thick  covering  of  glacial  drift.  Only  a  very  few  areas 
of  any  considerable  size  are  completely  free  from  drift;  these 
comprise  the  hilly  parts  of  east  Yorkshire,  and  the  lower  part 
of  the  Vale  of  York,  continuing  southwards  up  the  Trent  valley 
about  as  far  as  Nottingham.  It  is  manifestly  impossible  to 
give  a  full  account  of  all  the  types  of  glacial  deposit  found  even 
in  the  lowland  and  cultivated  parts  of  the  British  Isles,  and 
a  few  selected  examples  must  suffice. 

Yorkshire  and  Lincolnshire.  The  drifts  of  eastern  Yorkshire 
form  a  somewhat  complicated  series,  and  the  true  relationship 
of  the  different  members  is  not  easy  to  ascertain.  In  the 
northern  part  of  the  county  the  most  conspicuous  is  a  reddish 
brown  boulder-clay  of  quite  remarkable  tenacity.  This  spreads 
widely  over  the  low  ground,  and  fills  up  the  seaward  ends  of 
many  valleys.  Here  and  there  are  found  in  it  seams  of  sand 
or  gravel  and  boulders  of  all  sizes  are  abundant,  especially  those 
from  Scotland  and  the  Lake  District ;  particularly  notable  are 
the  great  boulders  of  Shap  granite  from  Westmorland,  sometimes 
over  a  ton  in  weight.  This  clay  gives  rise  to  a  very  heavy 
soil,  much  of  it  being  rather  boggy  pasture ;  when  well  drained 
it  is  fertile  and  can  carry  heavy  crops.  South  of  Flamborough 
Head,  in  the  low-lying  district  of  Holderness,  several  distinct 
clays  can  be  recognized,  overlain  in  some  places  by  extensive 


xvi]  RECENT  FORMATIONS  297 

spreads  of  gravel.  The  total  thickness  of  all  the  deposits  is 
about  100  feet.  The  boulder-clay  forms  a  stiff  rich  marly  soil, 
which  however,  owing  to  special  local  causes,  varies  somewhat 
in  texture  from  place  to  place.  The  more  elevated  parts  of  the 
boulder-clay  are  covered  by  a  loamy  soil,  not  specially  heavy, 
and  Mr  Clement  Reid1  attributes  this  fact  largely  to  the  action 
of  the  wind  which  has  blown  away  the  finer  particles  of  clay, 
thus  lightening  the  soil.  This  fine  material  is  again  deposited 
in  more  sheltered  localities,  thus  making  the  clays  locally  still 
heavier  and  stifTer,  but  occasionally  improving  sandy  patches 
(see  also  p.  139).  The  boulder-clay  soils  of  Holderness  are 
naturally  good  fertile  wheat  lands,  but  of  late  years  much  has 
been  laid  down  to  grass.  The  alluvial  deposits  of  this  area  are 
described  elsewhere. 

Throughout  the  great  central  plain  of  Yorkshire,  from  the 
Tees  to  a  little  south  of  York,  there  is  a  vast  spread  of  glacial 
deposits.  Along  the  Tees  two  clays  can  be  recognized,  separated 
by  sands  and  gravel.  The  lower  boulder-clay  is  bluish  when 
fresh  and  contains  many  foreign  boulders,  often  well  striated; 
it  rises  high  on  the  flanks  of  the  Cleveland  and  Hambleton 
Hills.  The  upper  boulder-clay  is  reddish,  with  few  stones,  and 
it  does  not  ascend  nearly  so  high. 

In  Lincolnshire  there  are  two  quite  distinct  areas  of  boulder- 
clay.  On  the  east  of  the  Wolds  are  found  two  clays,  as  in 
Yorkshire ;  the  purple  clay  below  and  the  Hessle  clay  above, 
with  occasional  beds  of  sand  and  gravel.  The  purple  clay  is  a 
stiff  purplish-brown  clay  with  many  foreign  boulders,  while  the 
Hessle  clay  is  greyish- brown  and  sometimes  contains  a  little 
Chalk.  On  the  west  of  the  Wolds  most  of  the  low  country  is 
covered  by  the  chalky  boulder-clay,  whose  character  is  perhaps 
sufficiently  explained  by  its  name ;  sometimes  at  first  sight  this 
looks  like  Chalk  in  place,  but  enclosed  fragments  of  other  rocks 
soon  show  its  true  nature.  It  forms  part  of  the  great  sheet  of 
chalky  boulder-clay  that  covers  so  much  of  the  eastern  and 
midland  counties  almost  as  far  south  as  the  Thames. 

East  Anglia.     The  general  succession  of  the  glacial  deposits 

1  Reid,  "Geology  of  Holderness,"  Mem.  Geol.  Survey,  1885,  p.  115.     "Dust 
and  Soils,"  Geol.  Mag.  1884,  p.  165. 


298  THE   PLEISTOCENE   AND  [CH. 

of  Norfolk,  Suffolk  and  Cambridgeshire  has  already  been 
described  (p.  131).  Of  the  divisions  there  enumerated  only  two, 
the  glacial  sands  and  gravels,  and  the  chalky  boulder-clay,  are 
widespread  and  of  agricultural  importance,  the  others  being 
local.  The  soils  derived  from  the  glacial  formations  of  this 
district  show  an  enormous  amount  of  variation,  ranging  from 
the  barren  and  uncultivated  sands  of  the  Breckland  of  western 
Norfolk  to  heavy  clays  in  Cambridgeshire.  The  problem  is 
also  much  complicated  in  many  parts  by  the  occurrence  of 
different  kinds  of  brick-earths  and  silts  laid  down  in  still  water 
in  late  glacial  times;  the  exact  manner  of  origin  of  many  of 
these  is  still  a  mystery,  since  they  do  not  always  occur  on  the 
lowest  ground,  being  found  also  on  ridges  elevated  above  the 
general  level  of  the  surrounding  country.  Some  of  these  were 
formerly  mistaken  for  boulder-clay,  Gault  clay  and  other 
formations. 

In  Suffolk  the  boulder-clay  occupies  a  very  large  area;  it 
is  generally  very  chalky,  but  varies  much  in  character,  ranging 
from  good  corn  land  to  poor  heavy  clay.  Most  of  it  is  well 
suited  to  wheat  and  beans,  the  lighter  varieties  to  barley.  The 
glacial  sands  and  gravels  vary  from  land  capable  of  producing 
excellent  crops  of  barley  to  light  hungry  heath  land,  almost 
worthless  from  the  agricultural  point  of  view. 

The  Western  Midlands.  Another  area  of  glacial  deposits  of 
great  agricultural  importance  forms  the  wide  plains  of  south 
Lancashire  and  Cheshire,  with  extensions  into  North  Wales, 
Shropshire  and  Staffordshire.  Most  of  this  country  is  underlain 
by  Trias,  and  this  formation  has  an  important  influence  in 
determining  the  character  of  the  drifts  on  the  principle  already 
explained.  The  ice  apparently  moved  inland  from  the  Irish 
Sea,  ploughing  up  the  soft  Triassic  sandstones  and  marls  and 
incorporating  with  them  many  boulders  from  the  Lake  District 
and  southern  Scotland.  The  result  is  a  boulder-clay  of  a  pre- 
vailing red  tint,  with  a  strong  resemblance  to  the  local  Triassic 
deposits.  This  forms  heavy  soils,  mostly  under  grass  and 
devoted  to  dairying.  The  glacial  sands  and  gravels  on  the  other 
hand  generally  occupy  somewhat  higher  ground,  and  form  light 
sandy  soils,  often  with  a  pan  (locally  called  Fox-bench)  at  a 


xvi]  RECENT   FORMATIONS  299 

depth  of  4  to  14  inches.  This  is  clearly  analogous  to  the 
Ortstein  of  German  writers.  The  high-level  shell-bearing 
glacial  gravels  of  North  Wales  and  Cheshire  have  already  been 
mentioned. 

The  boulder-clays  and  glacial  gravels  of  the  western  Midlands 
are  of  very  varying  character  and  origin.  Boulders  from 
North  Wales  are  found  as  far  east  as  Birmingham,  while  rocks 
from  the  Lake  District  are  abundant  in  the  middle  part  of 
the  Severn  valley,  about  Shrewsbury.  All  over  the  western 
Midlands  the  drifts  contain  in  abundance  the  characteristic 
rounded  stones  from  the  Bunter  Pebble  Beds  of  Staffordshire 
and  Cheshire,  and  these  extend  about  as  far  south  as  Oxford. 
The  chalky  boulder-clay  of  eastern  type  also  appears  to  stretch 
westwards  into  Warwickshire  and  the  line  of  demarcation 
between  the  drifts  of  western  and  eastern  origin  is  uncertain. 
It  is  impossible,  owing  to  their  variety,  to  give  any  general 
account  of  the  drift  soils  of  these  regions ;  they  vary  from  stiff 
clays  to  light  sands,  according  to  the  character  of  the  under- 
lying or  immediately  contiguous  rocks,  which  belong  to  many 
different  formations  within  a  comparatively  small  area. 

In  the  north  of  Shropshire  drift  deposits  cover  a  large  area. 
They  include  sand  and  not  very  stiff  clays.  Most  of  the  latter 
yield  medium  loams  of  a  brownish  colour  that  can  grow  good 
wheat  and  oats,  but  there  is  much  grass,  especially  near 
Shrewsbury,  devoted  to  cattle- farming  and  dairying.  The  drift 
sands  form  light  soils,  sometimes  peaty  and  generally  deficient 
in  lime;  they  are  specially  good  for  potatoes  and  carrots. 

Northern  England  and  Scotland.  Glacial  deposits  of  various 
kinds  are  of  almost  universal  occurrence  in  the  valleys  and 
plains  of  northern  England  and  southern  Scotland.  During  the 
earlier  part  of  the  Pleistocene  period  the  mountains  gave  rise 
to  local  glaciers  which  left  their  deposits  on  the  lower  ground 
almost  everywhere  and  greatly  modifiecl  the  agricultural 
character  of  these  areas.  The  plain  of  north  Cumberland  and 
the  valley  of  the  Eden  were  overflowed  by  ice  coming  from  the 
Galloway  district,  bringing  innumerable  boulders  of  granite  and 
other  rocks.  These  drifts  are  of  great  thickness  and  render 
difficult  the  demarcation  of  the  underlying  rock-systems.  In 


300  THE   PLEISTOCENE   AND  [CH. 

the  Eden  valley  in  particular  the  peculiar  and  characteristic 
forms  of  the  glacial  deposits  are  well  marked,  especially  the  long 
narrow  ridges  commonly  known  as  drumlins,  which  lie  with 
their  long  axes  parallel  to  the  direction  of  movement  of  the  ice. 
They  consist  mostly  of  a  very  stony  boulder-clay,  with  occasional 
seams  of  gravel  and  sand.  Since  the  solid  rocks  below  belong 
to  the  Permian  and  Triassic  systems  the  glacial  deposits  are 
generally  of  a  red  colour.  Around  Carlisle  and  stretching  well 
into  Scotland  is  a  great  spread  of  alluvium,  often  of  a  peaty 
nature,  and  including  the  well-known  Sol  way  Moss.  The 
lower  ground  of  the  Southern  Uplands  is  also  for  the  most  part 
occupied  by  glacial  deposits,  formed  by  the  local  ice-streams, 
which  radiated  in  all  directions  from  the  higher  mountains  and 
especially  from  the  granitic  peaks  of  Galloway.  The  mixing  of 
soil-material  thus  brought  about  has  increased  the  fertility  of 
this  region,  which  is  now  a  district  of  good  farming  and  stock- 
breeding;  the  Galloway  and  Ayrshire  cattle  are  famous. 

In  northern  Scotland  the  evidences  of  glaciation  are  almost 
everywhere  very  conspicuous ;  the  mountains  show  the  charac- 
teristic features  of  ice  erosion,  and  the  valleys  and  plains  are 
occupied  by  great  thicknesses  of  boulder-clay  and  other  glacial 
deposits.  The  boulder-clays,  which  are  often  very  stony,  are 
interstratified  locally  with  beds  of  gravel,  sand  and  peat, 
completely  masking  the  underlying  solid  formations  and 
yielding  soils  of  the  most  varied  character,  generally  however 
somewhat  on  the  heavy  side.  The  richest  of  these  soils  are 
those  composed  of  debris  from  the  volcanic  rocks  of  Old  Red 
Sandstone  and  Carboniferous  age  that  cover  such  large  areas 
in  the  central  valley  of  Scotland.  Perhaps  best  of  all  are  the 
soils  of  the  Carse  of  Gowrie,  which  contain  much  material 
derived  from  the  lavas  of  the  Ochil  and  Sidlaw  Hills.  In  the 
highly  farmed  and  highly  rented  district  of  East  Lothian,  the 
soils  are  derived  very  largely  from  boulder-clays  resting  on 
strata  of  the  Old  Red  Sandstone  system,  most  of  the  material 
of  the  glacial  deposits  being  of  local  origin. 

The  soils  of  the  important  agricultural  district  of  Aberdeen- 
shire  are  almost  entirely  on  drift,  very  few  being  derived  from 
rock  in  place.  The  glacial  deposits  have  however  in  places  been 


xvi]  RECENT  FORMATIONS  301 

considerably  worked  over  and  modified  by  running  water, 
especially  in  the  river  valleys.  The  most  striking  characteristic 
of  the  soils  of  this  area  is  their  variability.  In  parts  there  are 
heavy  glacial  clays,  while  in  other  places  the  soils  are  very  light 
and  gritty.  Over  considerable  areas  also  the  soils  are  peaty. 
It  is  very  difficult  to  find  any  considerable  area  of  uniform  type 
and  no  useful  generalizations  can  be  drawn  up. 


CHAPTER   XVII 

THE   GEOLOGICAL  HISTORY   OF  THE   DOMESTIC  ANIMALS1 

A  work  on  Agricultural  Geology  could  scarcely  be  considered 
complete  without  some  attempt  to  state  what  is  known  con- 
cerning the  origin  and  geological  history  of  those  animals  which, 
by  reason  of  their  utility  in  the  service  of  Man,  have  ever 
been  a  source  of  interest,  especially  to  agriculturists.  The 
subject  is  one  of  great  complexity,  and  is  discussed  here  mainly 
from  the  standpoint  of  well-established  facts.  Nevertheless  the 
hypotheses  that  have  been  put  forward  are  full  of  interest  and 
have  served  to  draw  attention  to  the  importance  of  the  question. 

The  animals  dealt  with  are  the  Horse,  the  Ox,  the  Sheep, 
the  Pig  and  the  Dog.  In  the  case  of  some  of  these  species  the 
evidence  available  for  building  conclusions  is  of  the  scantiest, 
and  they  are  accordingly  dealt  with  here  with  corresponding 
brevity. 

THE  HORSE 

It  is  generally  agreed,  even  by  those  who  most  definitely 
adhere  to  the  view  that  our  modern  horses  and  ponies  have  had 
a  multiple  origin,  that  the  little  primitive  four- toed  Hyra- 
cotherium,  first  described  in  1839  by  Owen,  and  found  in  the 
London  Clay  of  Studd  Bay,  Herne,  Kent,  stands  very  near  to 
the  base  of  the  stem  from  which  all  the  equine  animals  sub- 
sequently arose.  The  American  palaeontologist,  Cope,  regarded 
the  still  older  five-toed  and  semi-plantigrade  Phenacodus  of  the 
lower  Eocene  of  South  America  as  a  more  remote  ancestor,  but 
this  animal  was  probably  too  large  and  in  certain  respects  too 

1  By  F.  H.  A.  Marshall,  Sc.D. 


CH.  xvn]     HISTORY   OF   DOMESTIC   ANIMALS  303 

specialized  to  have  been  in  the  direct  line  of  equine  descent. 
The  Hyracotheriidae  died  out  in  Europe  at  the  close  of  the 
Eocene  period ;  at  least  no  representatives  of  the  equine  family 
have  been  found  fossil  in  the  Oligocene  of  the  Old  World,  and 
we  have  to  pass  to  the  American  continent  to  trace  the  progress 
of  the  Horse's  descent.  The  record  shown  in  the  Tertiary 
strata  of  North  America  is  very  complete,  and  consequently  the 
inference  has  been  drawn  that  the  evolutionary  process  by  which 
the  five- toed  and  four-toed  ancestors  of  the  Horse  became  trans- 
formed into  the  modern  single- toed  Equus  took  place  exclusively, 
or  almost  exclusively,  upon  that  continent,  the  various  genera 
which  have  been  found  fossil  in  the  later  Tertiary  formations 
of  Europe  and  Asia  being  supposed  to  have  migrated  from  the 
New  World.  It  must  however  be  pointed  out  that  such  an 
inference  is  of  very  doubtful  validity,  since  the  absence  of  fossil 
Equidae  in  the  Oligocene  strata  of  Europe  and  Asia  is  only 
negative  evidence. 

In  North  America  Hyracotherium  was  represented  by  the 
closely  similar  Eohippus.  The  descent  of  the  modern  Horse 
from  this  genus  has  been  described  by  the  American  geologist 
Marsh,  whose  researches  on  equine  lineage  have  become  classical. 
He  thus  summarizes  the  results  of  his  investigations : 

"The  oldest  representative  of  the  Horse  at  present  known 
is  the  diminutive  Eohippus  from  the  Lower  Eocene.  Several 
species  have  been  found,  all  about  the  size  of  a  fox.  Like  most 
of  the  early  mammals,  these  ungulates  had  forty- four  teeth,  the 
molars  with  short  crowns  and  quite  distinct  in  form  from  the 
premolars.  The  ulna  and  fibula  were  entire  and  distinct,  and  there 
were  four  well-developed  toes  and  a  rudiment  of  another  on  the 
forefeet,  and  three  toes  behind.  In  the  structure  of  the  feet 
and  teeth,  the  Eohippus  unmistakably  indicates  that  the  direct 
ancestral  line  to  the  modern  Horse  has  already  separated  from 
the  other  perissodactyles,  or  odd-toed  ungulates. 

"In  the  next  higher  division  of  the  Eocene  another  genus, 
Orohippus,  makes  its  appearance,  replacing  Eohippus,  and 
showing  a  greater,  though  still  distant,  resemblance  to  the  equine 
type.  The  rudimentary  first  digit  of  the  forefoot  has  dis- 
appeared, and  the  last  premolar  kas  gone  over  to  the  molar 


304  THE   GEOLOGICAL  HISTORY  [CH. 

series.  Orohippus  was  but  little  larger  than  Eohippus,  and  in 
most  other  respects  very  similar.  Several  species  have  been 
found  [but  none  occur  later  than  the  Upper  Eocene.] 

"  Near  the  base  of  the  Miocene,  we  find  a  third  closely  allied 
genus,  Mesohippus,  which  is  about  as  large  as  a  sheep,  and  one 
stage  nearer  the  Horse.  There  are  only  three  toes  and  a 
rudimentary  splint  on  the  forefeet,  and  three  toes  behind. 
Two  of  the  premolar  teeth  are  quite  like  the  molars.  The 
ulna  is  no  longer  distinct  or  the  fibula  entire,  and  other  char- 
acters show  clearly  that  the  transition  is  advancing. 

"In  the  Upper  Miocene  Mesohippus  is  not  found,  but  in  its 
place  a  fourth  form,  Miohippus,  continues  the  line.  This  genus 
is  near  the  Anchitherium  of  Europe,  but  presents  several 
important  differences.  The  three  toes  in  each  foot  are  more 
nearly  of  a  size,  and  a  rudiment  of  the  fifth  metacarpal  bone 
is  retained.  All  the  known  species  of  this  genus  are  larger 
than  those  of  Mesohippus,  and  none  of  them  pass  above  the 
Miocene  formation. 

"The  genus  Protohippus  of  the  Lower  Pliocene  is  yet  more 
equine,  and  some  of  its  species  equalled  the  Ass  in  size.  There 
are  still  three  toes  on  each  foot,  but  only  the  middle  one, 
corresponding  to  the  single  toe  of  the  Horse,  conies  to  the  ground. 

"  In  the  Pliocene  we  have  the  last  stage  of  the  series  before 
reaching  the  Horse,  in  the  genus  Pliohippus,  which  has  lost  the 
small  hooflets,  and  in  other  respects  is  very  equine.  Only  in 
the  Upper  Pliocene  does  the  true  Equus  appear  and  complete  the 
genealogy  of  the  Horse,  which  in  the  post-Tertiary  roamed  over 
the  whole  of  North  and  South  America,  and  soon  after  became 
extinct.  This  occurred  long  before  the  discovery  of  the  con- 
tinent by  Europeans,  and  no  satisfactory  reason  for  the 
extinction  has  yet  been  given.  Besides  the  characters  I  have 
mentioned,  there  are  many  others  in  the  skeleton,  skull,  teeth, 
and  brain  of  the  forty  or  more  intermediate  species,  which 
show  that  the  transition  from  the  Eocene  Eohippus  to  the 
modern  Equus  has  taken  place  in  the  order  indicated1." 
In  addition  to  the  North  American  genera  of  Equidae 

1  Marsh,   "Introduction  and  Succession  of   Vertebrate  Life  in  America," 
Nature,  vol.  xvi,  1877. 


xvn]  OF  THE  DOMESTIC  ANIMALS  305 

referred  to  in  the  above  cited  quotation,  certain  horse-like 
animals  have  been  found  fossil  in  Europe,  and  these,  as  already 
indicated,  have  been  regarded  by  some  authorities  as  lateral 
offshoots  from  the  main  equine  stem,  which  after  attaining 
a  considerable  degree  of  specialization  subsequently  became 
extinct.  Prominent  among  these  is  the  Hipparion  of  the  Lower 
Pliocene  strata  of  Germany,  Spain,  Greece,  Persia,  China,  and 
other  parts  of  the  Old  World.  In  Hipparion  the  lateral  digits 
are  rather  longer  than  in  Protohippus,  but  they  barely  extend 
beyond  the  level  of  the  first  phalanx  of  the  middle  digit.  The 
most  striking  peculiarity,  however,  is  that  the  anterior  inner 
pillar  of  the  molar  teeth  is  entirely  surrounded  by  enamel,  and 
is  thus  separated  from  the  rest  of  the  crown,  in  this  respect 
being  markedly  different  from  the  condition  found  in  E  quits 
and  other  members  of  the  horse  family. 


Fig.  44.    A  left  upper  molar  of  a  Single-toed  (Equus),  A,  and  a  Three-toed  Horse 
(Hipparion),  B.     p,  anterior  pillar;  hy,  posterior  pillar.    (From  Lydekker.) 

Another  European  genus,  but  belonging  to  an  earlier  epoch, 
is  Anchitherium  which  seems  to  have  been  akin  to  the  American 
Miohippus.  It  has  been  found  fossil  in  the  Lower  Miocene 
fresh-water  deposits  of  Sansan,  in  France.  Anchitherium  was 
about  the  size  of  a  sheep,  and  the  lateral  toes  though  smaller  than 
the  middle  ones  still  probably  reached  to  the  ground.  The  yet 
older  Hyracolherium  has  already  been  referred  to. 

It  used  to  be  supposed  by  palaeontologists  that  the  horses 
and  ponies  which  we  know  to-day  were  descended  by  a  direct 
and  single  line  of  ancestry  from  Eohippus  or  Hyracotherium. 

R.A.G.  20 


306  THE   GEOLOGICAL  HISTORY  [CH. 

We  now  know  that  at  various  times  during  the  Tertiary  period 
there  were  several  kinds  of  horses  existing  contemporaneously 
and  that  in  some  cases  two  or  more  species  inhabited  the  same 
area.  In  Miocene  times  we  have  the  somewhat  coarse-limbed 
Hypohippus  which  was  apparently  adapted  for  living  in  forest 
areas,  and  the  comparatively  slender-limbed  Neohipparion 
which  was  specialized  for  a  desert  life,  both  of  these  horses 
having  been  found  in  North  America.  Hypohippus  is  of  special 
interest  in  that  it  had  a  vestige  of  the  first  metacarpal  bone, 
such  as  is  still  very  occasionally  found  in  coarse-limbed  breeds 
of  horses  but  never  in  slender-limbed  breeds.  Another  Miocene 
horse,  Merychippus,  was  fine-boned,  and,  according  to  Ewart1, 
bears  some  affinities  to  the  modern  "Celtic  pony"  or  "plateau" 
horse. 

The  first  to  suggest  that  modern  horses  had  a  multiple 
origin  was  Hamilton  Smith,  writing  in  the  middle  of  the  last 
century,  but  his  views  were  generally  disregarded.  In  recent 
years,  however,  Osborn2,  on  the  palaeontological  side,  Ewart, 
as  a  result  of  a  careful  study  of  existing  breeds  and  numerous 
cross-breeding  experiments,  and  Ridgeway,  from  the  standpoint 
of  the  archaeologist,  have  arrived  at  conclusions  which  without 
being  identical  agree  in  postulating  several  independent  origins 
for  the  domestic  horses  of  to-day. 

Ewart,  who  has  paid  more  attention  to  this  subject  than 
any  other  zoologist,  refers  to  three  different  Miocene  species  of 
the  genus  Equus  which  were  probably  concerned  in  the  formation 
of  existing  varieties  of  horses.  These  are  E.  sivalensis  of  the 
Indian  Pliocene,  E.  stenonis  of  the  Pliocene  deposits  of  Europe 
and  North  Africa,  and  a  species  named  by  Ewart  E.  agilis 
(=  Asinus  fossilis  of  Owen),  which  has  been  found  fossil  in  the 
Pliocene  formations  of  Italy  and  France,  as  well  as  in  Pleistocene 
deposits  of  North  Africa,  France,  and  England  (e.g.  in  a 
cavernous  fissure  at  Oreston,  near  Plymouth).  These  three 
species  agree  in  having  a  short  anterior  internal  pillar  (a  fold 
of  enamel  on  the  inner  surface  of  the  tooth)  on  the  premolars 

1  Ewart,  Appendix  to  The  Shetland  Pony,  by  C.  and  A.  Douglas,  Edinburgh 
and  London,  1913. 

2  Osborn,  The  Age  of  Mammals,  New  York,  1910. 


XVII] 


OF  THE  DOMESTIC   ANIMALS 


307 


^ x 


Fig.  45.  The  Ancestors  of  the  Horse  and  its  Relatives  compared  in  size  and 
form  with  their  typical  modern  representative,  a,  Hyracotherium  of  the 
Lower  Eocene ;  b,  Orohippus,  of  the  Middle  Eocene ;  c,  Mesohippus,  of  the 
Oligocene ;  d,  Merychippus.  of  the  Miocene ;  e,  Pliohippus,  of  the  Pliocene ; 
/,  the  Horse,  Equus  caballus.  (From  Lydekker.) 


20—2 


308  THE   GEOLOGICAL  HISTORY  [CH. 

and  first  molar.  They  differ  in  certain  other  characters,  and 
notably  in  the  size  and  deflection  of  the  face  and  in  the  limb 
bones1. 

E.  sivalensis  is  the  oldest  of  the  one-hoofed  horses  with  long 
(hypsodont)  molars,  and  the  largest  of  the  Old  World  fossil 
horses,  since  it  stood  about  15  hands  high.  It  had  long,  rather 
slender  limbs,  a  large  head,  a  tapering  face,  a  long  neck,  high 
withers,  and  a  tail  set  high  on  the  body.  It  is  found  fossil  in  the 
famous  Siwalik  deposits  of  the  Himalayas.  It  was  apparently 
adapted  for  a  life  in  upland  valleys.  According  to  Ewart,  it 
is  the  probable  ancestor  of  some  Indian  and  other  Oriental 
breeds  of  horses  (e.g.  certain  long-faced  Kirghiz  horses  with  a 
sloping  forehead  and  long  ears).  Lydekker  on  the  other  hand 
has  suggested  that  it  may  have  been  the  ancestral  stock  of 
Barbs,  Arabs,  and  Thoroughbreds2. 

In  E.  stenonis  the  anterior  pillars  of  the  premolars  and 
molars  were  shorter  than  in  E.  sivalensis.  This  species  was  not 
generally  so  big  as  the  Siwalik  Horse,  though  the  limb  bones 
found  fossil  seem  to  show  that  it  may  sometimes  have  reached 
15  hands.  The  metacarpal  bones  were  somewhat  thicker  than 
in  E.  sivalensis.  According  to  Ewart,  E.  stenonis,  which  was 
widely  distributed  over  Europe  and  North  Africa  in  Pliocene 
times,  probably  had  an  important  share  in  the  making  of  the 
modern  Shires  and  other  heavy  breeds,  the  latter  being  the 
modern  representatives  of  the  " forest"  type  called  by  Ewart 
Equus  robustus. 

The  Equus  agilis  of  Ewart  was  a  smaller  horse  with  slender 
limbs.  In  the  metacarpals  and  in  the  pillars  of  the  molar 
teeth  it  differed  but  little  from  the  Pliohippus  of  the  Miocene 
and  early  Pliocene  periods.  It  is  supposed  to  have  given  rise  to 
the  modern  Celtic  ponies  and  other  horses  of  the  "plateau" 
type  referred  to  below. 

Coming  to  Pleistocene  and  Recent  times  we  find  evidence 
of  the  existence  of  at  least  three  species  of  horses.  Of  these 

1  Ewart,  "The  possible  Ancestors  of  the  Horses  living  under  Domestication," 
Proc.  Royal  Soc.   B,  vol.  LXXXI,    1909.      "The   Origin   of    the   Clydesdale," 
Trans.  Highland  and  Agric.  Soc.  1911. 

2  Lydekker,  The  Horse  and  its  Relatives,  London,  1912. 


xviil 


OF   THE   DOMESTIC   ANIMALS 


309 


Fig.  46.  Bones  of  forefeet  of  extinct  forerunners  of  the  Horse.  A,  Hyracotherium  or 
Eokippus;  B,  Mesohippus;  C,  Merychippus  or  Protohippus ;  D,  Hipparion. 
(From  Lydekker.) 


310  THE   GEOLOGICAL  HISTORY  [CH. 

one  only,  the  Steppe  Horse  of  Mongolia,  or  E.  przevalskii,  still 
survives  in  the  wild  state. 

This  species  is  characterized  by  a  long  narrow  face,  large 
nasal  chambers,  a  "Roman-nose,"  a  long  pillar  on  the  premolar 
and  first  molar  teeth,  a  short  back,  clean  limbs,  close  hocks, 
elongated  hoofs,  large  hind- chestnuts,  and  a  mule-like  tail  set 
on  high  up.  It  is  highly  specialized  for  a  steppe  life.  The 
characteristic  colour  is  a  yellow  dun.  Horses  of  this  type  have 
been  found  fossil  in  the  Roman  camp  at  Newstead. 

In  the  forest  type  (E.  robustus,  Ewart)  the  face  is  short  and 
broad  and  in  a  line  with  the  cranium,  the  internal  pillar  on  the 
molars  is  large,  the  middle  metacarpal  is  short  and  wide,  the 
back  is  long,  the  hindquarters  are  rounded,  the  hoofs  are  broad> 
the  hind- chestnuts  are  well  developed,  and  the  tail  is  set  on 
low  down.  This  type  is  represented  among  modern  breeds  by 
the  Shire,  the  Gudbrandsdal  of  Norway,  and  other  heavy  cart- 
horse varieties.  Forest  horses  have  been  found  fossil  in  the 
Brighton  Elephant  Bed,  in  Kent's  Cavern  at  Torquay,  and  in 
the  much  more  recent  Roman  camp  at  Newstead.  The  Stone 
Age  deposits  at  Solutre  also  contain  remains  of  Forest  horses. 

In  the  "plateau"  horses  the  face  is  fine  and  tapering,  the 
head  is  typically  small,  the  pillars  of  the  molars  are  short,  the 
neck  is  long  and  the  back  short,  the  middle  metacarpals  are 
very  slender,  the  hoofs  are  small  and  the  hind-chestnuts  (hock 
callosities)  and  all  the  four  ergots  are  typically  wanting.  On 
the  upper  part  of  the  tail  there  is  a  fringe  of  short  hairs  forming 
a  tail-lock,  which  is  shed  at  the  beginning  of  summer.  The 
typical  colour  is  light  dun,  and  there  is  often  a  dorsal  stripe 
or  eel-mark  and  some  cross  stripes  on  the  legs1.  The  Celtic 
pony  is  represented  in  its  present  form  by  some  of  the  Iceland 
and  Faroe  ponies  and  by  the  Udganger  ponies  of  Norway.  It 
has  formed  the  basis  of  most  of  the  pony  breeds  of  the  world 
(including  such  varieties  as  the  Fjordhest  of  Norway),  besides 
having  contributed  to  the  formation  of  the  Clydesdale.  It  has 
been  suggested  that  the  "Celtic"  or  "plateau"  characters  of 
many  modern  British  breeds  are  due  to  infusion  of  Norwegian 

1  Ewart,  "The  Multiple  Origin  of  Horses  and  Ponies,"  Trans.  Highland  and 
Agric.  Soc.  1904. 


xvn]  OF   THE   DOMESTIC  ANIMALS  311 

blood  and  that  ponies  of  the  agilis  type  were  brought  over  not 
merely  to  Iceland  and  the  Faroes  but  also  to  Britain  between 
the  eighth  and  tenth  centuries1.  According  to  Ewart  E.  agilis 
had  at  one  time  a  wide  distribution,  for  the  Arab  and  the 
other  finely  built  horses  are  supposed  to  have  sprung  from  this 
stock.  Remains  of  the  "plateau"  or  Celtic  variety  have  been 
found  fossil  in  Kent's  Cavern  and  other  Palaeolithic  deposits, 
and  in  the  Roman  camp  at  Newstead2. 

It  remains  briefly  to  consider  Ridgeway's  views  concerning 
the  origin  of  the  breeds  from  which  the  Thoroughbred  has  been 
derived3.  According  to  this  author  there  were  two  chief 
varieties  which  have  contributed  in  varying  proportions  to 
the  formation  of  all  the  improved  breeds  of  horses.  The  first 
is  the  coarse,  large-headed  horse  of  Europe  and  Asia.  This 
animal  appears  to  have  been  the  same  as  the  Forest  Horse  of 
Ewart,  but  bore  a  relationship  to  E.  przevalslcii.  The  second 
was  the  African  variety  called  by  Ridgeway  E.  caballus  libicus 
which  was  characterized  by  a  fine  head,  a  star  on  the  forehead 
and  white  points,  a  tendency  to  striping,  a  tail  set  on  high  up, 
and  the  absence  of  hind-chestnuts.  This  type  is  supposed  to 
be  represented  by  the  better  class  of  Arabs  and  Barbs  at  the 
present  time,  and  to  have  been  the  foundation  stock  of  the 
Blood-horse  and  of  all  the  finer  types  of  horses  in  the  world ; 
nevertheless  in  giving  rise  to  these  it  underwent  varying  degrees 
of  admixture  with  the  large,  coarse-headed  variety  referred  to 
above.  Concerning  the  existence  of  the  Celtic  pony  as  a  separate 
sub-species  Ridgeway  is  doubtful,  and  he  appears  to  regard  this 
animal  as  probably  descended  also  in  part  from  a  Libyan 
ancestor. 

Ewart  has  pointed  out  that  skulls  of  the  Celtic  type  have  been 
found  at  Newstead,  and  that  these  closely  resemble  certain 
high-caste  Arabs,  and  this  fact,  together  with  other  evidence, 
has  led  him  to  infer  that  the  Celtic  pony,  the  Libyan  horses 

1  Marshall,  "The  Horse  in  Norway,"  Proc.  Royal  Soc.  Edin.  vol.  xxvi,  1905. 

2  Ewart,    "On   Skulls   of   Horses   from   the   Roman   Fort   at   Newstead," 
Trans.  Royal  Soc.  Edin.  vol.  XLV,  1907. 

3  Ridgeway,  The  Origin  and  Influence  of  the  Thoroughbred  Horse,  Cambridge, 
1905 


312  THE   GEOLOGICAL  HISTORY  [CH. 

of  the  second  century,  and  the  better  type  of  modern  Arab, 
are  all  derived  from  the  same  ancestral  stock. 

Without  expressing  a  complete  adherence  to  either  of  the 
theories  enunciated  above,  it  may  be  permitted  to  say  in  con- 
clusion that  there  appears  to  be  a  considerable  body  of  definite 
evidence  derived  from  various  sources — partly  palaeontological, 
partly  historical,  and  partly  experimental — in  favour  of  the 
view  that  the  various  breeds  of  horses  and  ponies  as  we  know 
them  to-day  have  had  a  multiple  origin,  and  that  in  all 
probability  more  than  two  species  of  equine  animals  have 
contributed  to  their  formation. 


THE  Ox 

There  are  few  data  supplied  by  geological  study  on  which  to 
formulate  definite  conclusions  concerning  the  origin  and  develop- 
ment of  existing  varieties  of  domestic  cattle.  Such  evidence 
as  is  available  for  this  purpose  has  been  procured  chiefly  from 
archaeological  and  historical  research,  and  in  view  of  the 
meagreness  of  the  information  obtained  it  is  not  remarkable 
that  the  views  which  have  been  put  forward  differ  widely  on 
certain  points. 

According  to  Zittel  the  Ox  and  all  other  hollow- horned 
Ruminants,  as  well  as  the  whole  tribe  of  deer,  had  a  common 
ancestor  in  a  small  Chevrotain-like  animal  named  Gelotes,  in 
which  the  radius  and  ulna  of  the  foreleg  and  the  tibia  and  fibula 
of  the  hindleg  were  well-developed  separate  bones  and  not 
unequally  developed  and  united  as  in  the  modern  Ox  or  Sheep. 
Gelotes,  which  is  found  fossil  in  Eocene  and  Oligocene  beds,  is 
supposed  to  be  descended  from  a  still  more  generalized  type  of 
Artiodactyl  Ungulate,  Pantolestes,  from  the  Lower  Eocene  of 
North  America. 

In  the  Pliocene  formations  several  species  or  varieties  of 
Bos  have  been  found  fossil,  often  associated  with  Bison.  The 
earliest  known  Bovidae,  like  the  Chevrotains,  were  probably 
hornless,  but  in  the  Lower  Pliocene  remains  have  been  found 
of  cattle  which  are  believed  to  have  been  horned  in  the  male, 
but  hornless  in  the  female.  Leptobos  falconeri  of  India  and 


xvn]  OF  THE  DOMESTIC  ANIMALS  313 

L.  etruscus  and  L.  lutus  of  Italy,  all  Lower  Pliocene  species,  are 
probably  instances  of  the  occurrence  of  this  condition1.  The 
remarkably  short  skull  is  another  characteristic  of  the  genus. 

The  genus  Bos  appears  to  have  been  represented  in  Pliocene 
times  by  two  or  three  species  found  fossil  in  the  Siwalik  Hills  of 
India.  Of  these  Bos  planifrons  (so-called  from  its  flattened 
forehead)  is  supposed  by  Duerst  to  have  been  the  ancestor  of 
B.  primigenius  and  B.  namadicus,  from  both  of  which  it  differed 
but  little.  According  to  another  view  Bos  acutifrons  (named  by 
Lydekker  after  the  sharp  ridge  running  longitudinally  down  the 
forehead2)  from  the  Siwalik  Pliocene  was  the  predecessor  of 
B.  namadicus.  It  seems  probable  that  all  these  forms  were 
closely  related,  and  that  whereas  the  sub-species  which  extended 
westward  became  B.  primigenius,  the  variety  which  remained 
and  spread  in  the  East  was  B.  namadicus.  This  species  is  found 
fossil  in  the  Pleistocene  gravels  of  the  Narbada  Valley  in  Central 
India.  It  approximates  somewhat  to  the  Gaur  and  Banting  in 
frequently  having  the  horn-cores  compressed  at  the  base. 

Bos  primigenius,  which  is  identified  with  the  Urus  of  Caesar, 
occurs  fossil  in  Pleistocene  deposits  in  the  British  Isles  and 
various  parts  of  Europe.  Its  remains  have  been  found  in 
Scottish  bogs  and  East  Anglian  fens,  in  alluvial  and  lacustrine 
deposits,  in  association  with  bones  of  the  Elephant,  the  Hippo- 
potamus, the  Rhinoceros  and  other  mammals.  Its  first  definite 
appearance  was  in  early  Palaeolithic  times,  when  it  lived 
alongside  the  Bison,  though  fossil  remains  which  have  been 
referred  by  some  investigators  to  the  Urus-have  been  discovered 
in  the  Upper  Pliocene  of  Germany  and  other  parts  of  Europe. 
It  seems  probable  that  the  Urus  first  arrived  in  Europe  just 
before  the  beginning  of  the  Glacial  period.  The  Bison  died  out 
at  the  end  of  the  Palaeolithic  period  but  the  Urus  survived, 
for  the  latter  animal  is  especially  characteristic  of  the  Neolithic 
Age.  The  following  is  Nilsson's  description  of  the  Urus: 
"The  forehead  [is]  flat;  the  edge  of  the  neck  straight,  the  horns 

1  Ewart,  "On  the  Skulls  of   Oxen  from  the  Roman  Military  Station  at 
Newstead,  Melrose,"  Proc.  Zool.  Soc.  1911. 

2  Lydekker  subsequently  expressed  himself  as  doubtful  about  the  specific 
validity  of  this  animal.     See  The  Ox  and  its  Kindred,  London,  1912. 


314  THE   GEOLOGICAL  HISTOEY  [CH. 

very  large  and  long,  near  the  roots  directed  outward,  and  some- 
what backward;  in  the  middle  they  are  bent  forward,  and 
towards  the  points  turned  a  little  upward.  This  colossal  species 
of  Ox,  to  judge  from  the  skeleton,  resembles  almost  the  tame 
Ox  in  form  and  the  proportions  of  its  body ;  but  in  its  bulk  it 
is  far  larger.  To  judge  from  the  magnitude  of  its  horn-cores  it 
had  much  larger  horns,  even  larger  than  the  long-horned  breed 
of  cattle  found  in  the  Campania  of  Rome.  According  to  all 
accounts,  the  colour  of  this  Ox  was  black ;  it  had  white  horns,, 
with  long  black  points ;  the  hide  was  covered  with  hair  like  the 
tame  Ox,  but  it  was  shorter  and  smooth,  with  the  exception  of 
the  forehead,  where  it  was  long  and  curly1." 


Fig.  47.     Skull  of  Urus.     (From  Lydekker.) 

McKenny  Hughes2  has  described  the  Urus  as  "a  large, 
gaunt  beast  with  a  long,  narrow  face."  Fleming3  mentions  a 
skull  in  his  possession  which  was  27J  inches  in  length  and 
11 J  inches  across  the  orbits,  and  Owen4  refers  to  a  skull  which 

1  Nilsson,  "On  the  Extinct  and  Existing  Bovine  Animals  of  Scandinavia," 
Annals  and  Mag.  of  Nat.  Hist.  vol.  TV,  2nd  series.  1849. 

2  Hughes,  "On  the  most  Important  Breeds  of   British  Cattle,"  Archaeo- 
logia,  vol.  LV,  1896. 

3  Fleming,  History  of  British  Animals,  London,  1828. 

4  Owen,  British  Fossil  Mammals,  London,  1846. 


xvn]  OF  THE  DOMESTIC  ANIMALS  315 

was  a  yard  in  length.  That  the  Urus  was  of  great  size  is  proved 
abundantly;  nevertheless  Caesar's  account  of  its  dimensions 
must  be  regarded  as  much  exaggerated.  The  following  is  his 
description  of  the  animals:  "In  size  they  are  a  trifle  smaller 
than  elephants;  in  kind,  colour,  and  shape  they  are  bulls. 
Great  is  their  strength  and  great  their  speed ;  nor,  having  espied 
them,  do  they  spare  either  men  or  beasts.  They  are  sedulously 
captured  in  pits  and  slain;  the  young  men  hardening  them- 
selves by  such  toil  and  training  themselves  by  this  kind  of 
sport ;  and  they  who  have  killed  most  Uri,  proclaimed  as  such 
by  the  horns  being  exhibited  in  public,  receive  great  commen- 
dation. But  it  is  not  possible  to  accustom  the  Uri  to  men  or 
to  tame  them,  not  even  though  they  are  caught  young.  Their 
horns  differ  much  in  size,  shape,  and  kind  from  those  of  our 
cattle1." 

Bos  frontosus,  so  named  by  Nilsson  because  it  had  a  mesial 
elevation  on  the  forehead,  was  in  other  respects  intermediate 
between  B.  longifrons  and  B.  primigenius.  Its  remains  have 
been  found  in  Britain.  A  local  variety  of  B.  primigenius,  found 
fossil  in  Tunis  and  Algeria,  has  been  called  B.  mauritanicus.  It 
had  a  relatively  short  forehead  and  somewhat  slender  limbs. 
Yet  another  sub-species  of  the  Urus,  called  B.  minutus  (since  it 
was  a  dwarf  variety),  has  been  discovered  in  the  superficial 
deposits  of  Belgium  in  association  with  the  Mammoth. 

Bos  longifrons,  which  is  identified  with  the  Celtic  Shorthorn, 
is  the  second  species  of  Ox  found  in  Britain,  Ireland  and  various 
parts  of  the  continent  during  the  Neolithic  Age.  According  to 
Hughes  it  is  the  only  breed  which  the  Romans  found  indigenous 
to  Britain  when  they  first  came  there.  It  has  been  found  fossil 
in  numerous  ancient  British  and  Roman  refuse  heaps  and 
rubbish  pits,  as  well  as  in  the  Swiss  Lake  Dwellings  and  in  other 
deposits  of  similar  age.  It  was  undoubtedly  a  domestic  variety. 
Bos  brachyceros,  remains  of  which  have  been  found  at  Anau  in 
Turkestan,  appears  to  have  been  the  same  as  B.  longifrons.  At 
the  present  day  it  is  probably  represented  in  its  purest  form 
by  the  Kerry  cattle.  Bos  longifrons  has  been  thus  described 
by  Owen:  "This  small  but  ancient  species  or  variety  of  Ox 
1  Caesar,  Gallic  War,  book  vi. 


316  THE   GEOLOGICAL  HISTORY  [CH. 

belongs,  like  our  present  cattle,  to  the  sub-genus  Bos,  as  is  shown 
by  the  form  of  the  forehead,  and  by  the  origin  of  the  horns  from 
the  extremities  of  the  occipital  ridge;  but  it  differs  from  the 
contemporary  Bos  primigenius,  not  only  by  its  great  inferiority 
of  size,  being  smaller  than  the  ordinary  breeds  of  domestic 
cattle,  but  also  by  the  horns  being  proportionately  much  smaller 
and  shorter,  as  well  as  differently  directed,  and  by  the  forehead 
being  less  concave.  It  is,  indeed,  usually  flat ;  and  the  frontal 
bones  extend  further  beyond  the  orbits,  before  they  join  the 
nasal  bones,  than  in  Bos  primigenius.  The  horn-cores  of  the 
Bos  longifrons  describe  a  single  short  curve  outwards  and  for- 
wards in  the  plane  of  the  forehead,  rarely  rising  above  that 
plane,  more  rarely  sinking  below  it;  the  cores  have  a  very 
rugged  exterior,  and  are  usually  a  little  flattened  at  their  upper 
part1."  McKenny  Hughes  says :  "  Bos  longifrons  was  a  very  small 
animal;  probably  not  larger  than  a  Kerry  cow.  It  was 
remarkable  for  the  height  of  its  forehead  above  its  orbits,  for 
its  strongly  developed  occipital  region,  and  its  small  horns 
curved  inward  and  forward2." 

It  has  been  shown  that  Bos  primigenius  probably  arose  from 
Bos  planifrons  in  the  East  and  that  Bos  namadicus  was  an 
Oriental  variety  of  B.  primigenius.  There  can  be  little  doubt 
that  Bos  longifrons  was  derived  originally  from  the  same  source, 
and  was  introduced  into  Britain  by  Neolithic  farmers  and 
herdsmen.  Duerst3  has  collected  evidence  of  some  importance 
concerning  the  history  of  Bos  longifrons  in  Turkestan  and  Meso- 
potamia. He  says  that  in  both  these  localities  unfavourable 
conditions  of  life  converted  "the  originally  large  and  stately 
Ox"  (Bos  macroceros),  which  was  longhorned  and  was  derived 
from  the  Asiatic  Urus  (B.  namadicus),  into  a  stunted,  short- 
horned  form,  the  Bos  brachyceros  or  longifrons.  The  first 
remains  of  Bos  macroceros  at  Anau  represent  oxen  which  lived 
about  8000  B.C.  By  about  6000  B.C.  this  animal  had  become 
replaced  by  B.  brachyceros.  Thus  the  longhorned  variety  of 

1  Owen,  loc.  cit. 

2  Hughes,  loc.  cit. 

3  Duerst,  "Animal  Remains  from  Excavations  at  Anau,"  in  Pumpelly's 
Exploration  in  Turkestan.     Carnegie  Institution  of  Washington,  vol.  n,  1908. 


xvn]  OF  THE  DOMESTIC  ANIMALS  317 

the  early  Babylonians  gave  rise  to  the  small  shorthorned  breed 
of  Assyrian  and  later  times.  Duerst  says  further  that  there  is 
reason  to  believe  that  the  Ox  of  Anau,  which  was  undergoing 
this  change,  eventually  reached  Central  Europe  after  migrating 
through  Southern  Russia  and  Eastern  Europe,  while,  as  already 
mentioned,  it  may  be  supposed  to  have  been  transported  to 
Britain  by  the  Neoliths.  The  longhorned  breed,  however, 
survived  without  much  alteration  in  Babylonia  and  Egypt, 
since  there  is  evidence  of  its  existence  in  these  countries  about 
4000  to  3000  B.C.,  as  well  as  in  India  and  China,  whence  it  was 
spread  by  tribal  migrations. 

It  remains  to  consider  the  origin  of  certain  of  our  modern 
breeds  of  cattle  and  the  different  views  which  have  been  put 
forward  as  to  their  evolution  and  history.  This  subject  can 
only  be  dealt  with  very  briefly,  for  the  evidence  which  has  been 
presented  is  chiefly  historical  rather  than  geological. 

According  to  some  authorities  the  "wild  cattle"  now  living 
in  Great  Britain,  that  is  to  say  the  Chillingham,  Cadzow  and 
Chartley  breeds,  are  the  descendants  of  the  Urus  which  was 
domesticated  on  the  continent  in  the  Neolithic  Age  and  trans- 
ported to  Britain  by  the  English  or  by  the  Scandinavian 
Vikings.  This  is  the  view  put  forward  by  Boyd  Dawkins1,  who 
points  out  that  there  is  no  evidence  of  any  large  domesticated 
cattle  existing  in  Britain  before  the  arrival  of  the  English.  On 
the  other  hand  it  has  been  suggested  that  the  Urus  may  have 
survived  in  Scotland  in  sufficient  numbers  to  have  given  rise 
to  the  Atholl  and  other  Scottish  "wild"  cattle,  but  there  is  no 
evidence  in  favour  of  such  a  view.  For  although  the  Urus  was 
widely  distributed  in  Great  Britain  in  the  Neolithic  period  it 
would  appear  probably  to  have  become  extinct  (at  least  in 
England)  by  the  time  of  the  Roman  invasion. 

Such  considerations  have  led  McKenny  Hughes  to  believe 
that  Bos  primigenius  has  had  no  share  whatever  in  forming 
either  the  Chillingham  or  any  other  British  breeds  of  cattle. 
This  geologist  supposes  the  Chillingham  breed  to  be  descended 
from  cattle  imported  by  the  Romans,  pointing  out  that  there 

1  Dawkins,  "  Chartley  White  Cattle."  Trans.  North  Staffordshire  Field  Club, 
vol.  xxxm,  1899. 


318  THE   GEOLOGICAL  HISTORY  [CH. 

is  a  close  resemblance  between  the  Chillinghams  as  they 
exist  to-day  and  certain  modern  Italian  cattle.  He  derives 
further  evidence  from  contemporary  drawings  of  Roman 
and  Ancient  Egyptian  cattle.  Against  this  view,  which  has 
been  adopted  by  James  Wilson1,  it  has  been  pointed  out  that 
there  is  no  evidence  that  the  Romans  imported  cattle  into 
Britain,  that  it  is  unlikely  that  they  would  have  done  so,  having 
regard  to  the  fact  that  domestic  cattle  already  existed  in 
Britain,  and  in  view  of  the  difficulties  of  transporting  animals 
in  sufficient  numbers  to  have  given  rise  to  English  breeds. 

Hughes  has  supposed  further  that  the  imported  Roman 
breed  blended  with  a  certain  admixture  of  Celtic  Shorthorn 
(already  present  in  the  country)  to  give  rise  to  the  Highland 
and  Welsh  cattle.  The  mediaeval  Shorthorn  is  regarded  as  a 
reversion  towards  the  native  variety  after  the  withdrawal  of  the 
Roman  legionaries  when  cattle  breeding  was  no  longer  carried 
on  by  the  same  careful  selection  as  formerly.  Lastly,  the 
longhorned  breeds  Hughes  shows  probably  to  be  descended 
from  Holstein  cattle  imported  in  the  Middle  Ages2. 

Cossar  Ewart3  has  shown  that  in  the  Roman  camp  at  New- 
stead  the  remains  of  five  fairly  distinct  types  of  oxen  occur  in 
addition  to  animals  that  were  probably  crossbred.  These  five 
types  are  as  follows :  (1)  The  Celtic  Shorthorn.  (2)  A  longhorned 
type  allied  to  the  Urus.  Such  animals  on  Hughes's  theory  were 
presumably  imported  by  the  Romans.  (3)  Oxen  with  deep 
notches  below  the  horn-cores  and  with  the  hind  view  of  the 
skull  generally  bearing  a  close  resemblance  to  Bos  acutifrons  of 
the  Siwalik  Pliocene.  It  is  possible  that  this  type  may  have 
been  of  the  nature  of  a  reversion,  brought  to  the  surface  by 
cross-breeding,  since  there  are  some  reasons  for  supposing  that 
B.  acutifrons  was  ancestral  to  B.  primigenius  or  at  least  to 
B.  namadicus.  (4)  Oxen  with  a  convex  forehead  and  horns 
curving  backwards  and  downwards,  and  with  some  affinities  to 
B.  namadicus.  These  also  may  in  certain  characters  have  been 
reversionary,  since  there  is  no  evidence  that  Bos  namadicus 

1  Wilson,  The  Evolution  of  British  Cattle,  London,  1909. 

2  Hughes,  loc.  cit. 

3  Ewart,  loc.  cit. 


xvn]  OF   THE   DOMESTIC   ANIMALS  319 

extended  westwards.  (5)  Hornless  cattle,  some  of  which  had 
a  mesial  prominence  and  therefore  belonged  to  the  frontosus 
type  of  Nilsson. 

The  question  as  to  the  origin  of  hornless  cattle  is  one  to 
which  no  answer  can  be  given.  Ewart  has  pointed  out  that 
such  cattle  existed  in  Egypt  under  the  Fourth  Dynasty,  and 
that  according  to  Tacitus  a  hornless  breed  lived  in  Germany 
in  the  first  century.  It  has  already  been  mentioned  that 
species  of  Leptobos  living  in  Pliocene  times  were  almost  certainly 
hornless  in  the  female.  It  may  be  that  a  reduction  in  the  horns 
took  place  gradually,  or  it  may  have  occurred  suddenly  through 
a  "mutation."  The  disappearance  of  horns  may  have  taken 
place  again  and  again  in  the  developmental  progress  of  the 
different  breeds,  and  so  may  be  regarded  (in  certain  cases  at 
least)  as  of  the  nature  of  a  reversion  to  a  previously  existing 
type.  These  are  problems  concerning  which  there  is  little 
evidence  forthcoming  towards  a  solution.  There  are,  however, 
records  of  horned  cattle  producing  hornless  "sports."  James 
C.  Lyell1  has  put  forward  the  theory  that  the  Polled  Angus 
Oattle  were  brought  from  Norway,  and  James  Wilson2  has 
contended  that  all  the  hornless  breeds  of  Britain  have  had  a 
Scandinavian  origin. 

It  will  be  seen  from  the  foregoing  account  that  whereas  the 
origin  of  the  various  types  of  domestic  cattle  is  still  a  subject 
of  much  controversy,  the  available  evidence  points  to  the  con- 
clusion that  they  were  all  derived  in  the  first  instance  from 
some  species  identical  with  or  closely  similar  to  the  wild  Urus 
or  B.  primigenius,  which  was  probably  first  domesticated  some- 
where in  Asia.  The  humped  cattle  of  India  (Bos  indicus) 
however  apparently  had  a  different  origin,  being  descended  from 
an  animal  resembling  the  Javan  Banting  (Bos  sondaicus), 
while  the  Gayal  or  domesticated  Ox  of  certain  Indo-Malay  and 
Indo-Chinese  races  is  not  markedly  different  from  the  wild 
Gaur  (B.  gaurus)  from  which  it  is  probably  derived3.  The 
history  of  these  species  and  of  the  domesticated  Buffalo  lies 
outside  the  scope  of  this  volume. 

1  Lyell,  The  Polled  Cattle  of  Angus,  1882.  2  Wilson,  loc.  cit. 

3  Lydekker,  loc.  cit. 


320  THE   GEOLOGICAL  HISTORY  [CH. 


THE  SHEEP 

The  tendency  of  opinion  among  zoologists  in  recent  years 
has  been  towards  assuming  that  the  Domesticated  Sheep,  like 
the  Horse,  has  had  a  multiple  origin,  and  that  of  the  four  main 
types  of  wild  sheep  now  in  existence,  namely  the  Mouflon,  the 
Urial,  the  Argali,  and  the  Bighorn,  two  if  not  three  have  been 
concerned  in  the  ancestry  of  the  domesticated  breeds.  More- 
over, there  are  strong  reasons  for  supposing  that,  as  in  the 
case  of  the  Ox,  the  process  of  domestication  first  took  place  in 
Asia,  the  domesticated  varieties  being  introduced  by  the 
ancestors  of  the  Swiss  Lake  Dwellers  into  Europe,  where  they 
have  since  undergone  great  variation  by  cross-breeding. 

The  origin  of  the  genus  Ovis  in  geological  times  is  a  problem 
that  is  still  more  obscure,  and  fossil  remains  of  sheep-like  animals 
have  done  little  to  elucidate  it.  The  Pliocene  Criotherium 
from  the  Isle  of  Samos,  an  animal  with  strange  twisted  horns, 
had  certain  affinities  to  the  Sheep,  but  it  is  not  supposed  to  have 
been  in  the  direct  line  of  descent.  A  similar  statement  may  be 
made  about  the  extinct  antelopes  of  the  genus  Oioceros  which 
have  been  found  fossil  in  the  Lower  Pliocene  of  Greece,  and  had 
spiral  horns  twisted  after  the  manner  of  sheep. 

Coming  to  more  recent  times  a  portion  of  the  skull  of  a 
sheep  has  been  found  in  the  Cromer  Forest  Bed.  In  this  animal 
the  curvature  of  the  horns  was  like  that  of  the  Mouflon,  but 
certain  other  characters,  and  more  particularly  the  size,  are 
suggestive  of  an  animal  allied  to  the  Urial.  The  Forest  Bed 
Sheep  has  been  called  0.  savini.  It  lived  in  Britain  contem- 
poraneously with  the  Elephant  and  the  Rhinoceros,  and  then 
died  out. 

The  Argali  type  was  represented  in  Pleistocene  times  by 
0.  antiqua  (in  the  south  of  France),  0.  argaloides  (in  Moravia) 
and  0.  ammon  fossilis  (in  Transbaikalia). 

Sheep  belonging  to  the  Bighorn  and  Ami  types  have  also 
been  found  fossil  in  Pleistocene  times. 

Of  the  types  now  in  existence  the  Bighorn  of  America  has 
never  been  domesticated,  and  the  Arui  of  the  same  continent 


xvii]  OF   THE   DOMESTIC   ANIMALS  321 

is  an  aberrant  species  which  has  sometimes  been  referred  to  a 
separate  genus. 

It  remains  therefore  to  consider  the  Mouflon,  the  Urial,  and 
the  Argali  types,  all  of  which  have  been  supposed  to  have  had 
a  share  in  giving  rise  to  one  or  other  of  our  domesticated 
varieties. 

The  Mouflon  at  present  inhabits  certain  parts  of  western 
Asia  and  various  islands  in  the  Mediterranean.  One  species 
(0.  musimon)  inhabits  Corsica  and  Sardinia.  Another  closely 


Fig.  48.     Skull  of  Sardinian  Mouflon  Ram  with  normal  horns.     (From  Ewart.) 

allied  but  smaller  species  (0.  orientalis)  lives  in  Cyprus,  and 
there  are  several  related  species  or  varieties  in  south-west  Asia. 
In  general  appearance  the  Mouflon  is  more  antelope-like  than 
any  domestic  sheep,  and  the  resemblance,  which  is  due  largely  to 
its  light,  neat  build,  is  increased  further  by  its  coat  of  short  hair. 
The  tail  is  short  and  the  head  comparatively  long  with  a  flat 
forehead,  and  in  the  male  large  spirally  coiled  horns  with  their 
tips,  in  normal  individuals,  directed  forwards.  The  limbs  are 
long  and  slender  and  terminate  in  sharp-edged  hoofs. 

The  Urial  (0.  vignei)  is  found  in  Turkestan  and  Afghanistan 
and  the  neighbouring  countries  as  far  east  as  the  Punjab  and 
Tibet,  where  it  lives  at  an  elevation  of  14,000  feet.  The  skeleton 

R.A.G.  21 


322  THE   GEOLOGICAL  HISTORY  [CH. 

is  similar  to  that  of  the  Mouflon,  but  the  large  and  deep  pit  on 
the  front  of  the  orbit,  where  the  face  gland  is  situated,  is 
characteristic  of  the  Urial.  The  horns  form  a  very  close  spiral. 
In  the  female  there  are  small  upright  goat-like  horns. 

The  Argali  (0.  ammon)  is  the  largest  of  all  the  wild  sheep, 
being  sometimes  as  high  as  13  hands  at  the  withers.  It  is 
found  mainly  on  the  mountain  ranges  around  the  Gobi  deserts, 
and  the  largest  variety  inhabits  the  region  of  the  Great  Altai 
Mountains.  The  horns  are  frequently  extraordinarily  massive 
and  spirally  coiled  as  in  the  Mouflon.  In  typical  specimens  the 
horns  of  the  old  rams  form  more  than  one  complete  circle, 
their  tips  reaching  considerably  beyond  the  lateral  margins, 
while  in  front  they  are  rounded  and  almost  touch  the  sides  of 
the  face.  They  may  reach  a  length  of  62  inches  and  a  girth  of 


Fig.  49.     Skull  and  horns  of  Altai  Ammon.     (From  Ewart.) 

20  inches.  In  some  kinds  of  Ammon  the  horns  are  "nipped 
in"  like  those  of  the  Scottish  Blackfaced  Sheep,  while  the 
horns  of  other  varieties  are  like  those  of  Merino  rams. 

It  is  now  generally  admitted  that  both  the  Mouflon  and  the 
Urial  have  taken  part  in  forming  some  of  the  breeds  of  domestic 
sheep.  The  half  wild  sheep  of  Soay  show  a  resemblance  to 
both  these  types,  sometimes  to  one  type  much  more  than  the 
other.  The  Shetland  variety  of  peat  sheep  and  the  sheep  whose 
remains  were  found  in  the  Roman  camp  at  Newstead  have 
been  shown  by  Ewart1  to  be  of  the  Urial  type.  Moreover  the 
Turbary  sheep  (0.  aries  palustris)  of  the  oldest  Swiss  Lake 
Dwellings  were  essentially  Urials  like  the  sheep  domesticated 

1  Ewart,  "Domestic  Sheep  and  their  Wild  Ancestors,"  Trans.  Highland  and 
Agric.  So*',.  1913  and  1914. 


xvn]  OF   THE   DOMESTIC  ANIMALS  323 

by  the  Anauli  in  Turkestan,  whence  they  were  carried 
westwards.  Elwes1  believes  that  the  diminutive  sheep  still 
existing  in  the  Orkneys  are  probably  derived  from  the  Turbary 
Sheep.  According  to  Lydekker2  there  is  nothing  in  the  physical 
characteristics  "to  preclude  all  the  British  long- tailed  sheep 
being  descended  from  the  Mouflon,  all  the  horned  breeds  having 
horns  of  the  general  type  of  those  of  the  Mouflon,"  but  he 
admits  the  probability  of  a  considerable  admixture  of  Urial 
blood  in  the  British  domesticated  breeds. 


Fig.  50.     Skull  and  horn-cores  of  Bronze  Age  Sheep,  according  to  Ewart  by 
origin  partly  Ammon.     (From  a  photograph  by  Prof.  Ewart.) 

Ewart  is  of  opinion  that  in  addition  to  the  Mouflon  and  the 
Urial,  the  Argali  sheep  have  had  a  share  in  forming  certain 
breeds,  and  in  support  of  this  view  he  cites  the  work  of  Douglas 
Carruthers3,  who  has  shown  that  the  tribes  of  Central  Asia, 
both  in  ancient  and  modern  times,  have  constantly  crossed 

1  Elwes,  Scottish  Naturalist,  1912. 

2  Lydekker,  The  Sheep  and  its  Cousins,  London,  1912. 

3  Carruthers,  Unknown  Mongolia,  London,  1913. 


324 


THE   GEOLOGICAL  HISTORY 


[CH. 


their  domesticated  ewes  with  wild  rams  of  the  Argali  or  Ammon 
type.  Ewart  adduces  further  evidence  based  on  the  examina- 
tion of  remains  from  Pleistocene  deposits,  on  a  comparison 
between  the  skeletons  of  wild  species  and  primitive  and  modern 
breeds,  and  on  cross-breeding  experiments,  in  support  of  the 
view  that  the  Scottish  Blackfaced  Sheep,  the  Merino,  and  possibly 
other  British  breeds,  are  in  part  descended  from  the  Argali1. 

After  the  beginning  of  the  Ice  Age,  when  0.  savini  became 
extinct,  there  were  no  sheep  in  Britain.  They  reappeared  with 
the  coming  of  the  Neoliths,  who,  as  we  have  already  seen, 
brought  with  them  their  domestic  animals,  and  among  these 


Fig.  51.     Skull  of  Dorset  Ram  showing  horns  of  Ammon  type.     (From  Ewart.) 

were  sheep  of  the  Mouflon  and  Urial  types.  Moreover,  Ewart 
has  shown  that  some  of  their  sheep  had  horns  like  those  of  a  small 
Ammon,  and  he  points  out  further  that  the  remains  of  sheep 
found  in  the  alluvium  of  the  Thames  Valley  show  a  marked 
resemblance  to  the  Argali  type. 

THE  PIG 

The  family  to  which  the  Pig  belongs  is  the  least  altered  or 
most  primitive  of  the  hoofed  animals.  They  differ  from  horses, 
sheep  and  oxen  in  having  the  ulna  of  the  forelimb  and  the 

1  Ewart,  loc.  cit. 


xvn]  OF  THE  DOMESTIC  ANIMALS  325 

fibula  of  the  hindlimb  as  separate  bones,  distinct  from  the  radius 
and  the  tibia.  The  bones  of  the  wrist  and  ankle  (carpals  and 
tarsals)  are  not  fused  together.  The  molar  teeth  are  bunodont, 
that  is  to  say  their  crowns  bear  simple  or  roundish  bosses  or 
tubercles,  and  not  crescent  shaped  structures  as  in  Ruminants, 
or  complicated  irregular  ridges  as  in  horses.  The  canine  teeth 
are  well  developed  and  the  complete  mammalian  dentition  of 
forty-four  teeth  is  generally  present. 

We  find  the  above-mentioned  characters  in  Homacodon  of 
the  Middle  Eocene  of  Wyoming,  U.S.A.  This  animal  was  no 
larger  than  a  rabbit  and  had  five  digits,  but  it  was  essentially  a 
primitive  pig.  Cheropotamus,  of  the  Upper  Eocene  of  the  Isle 
of  Wight  and  France,  was  larger  but  not  very  dissimilar.  The 
Miocene  Hyotherium  which  ranged  over  Europe  and  Asia  was 
as  large  as  the  Wild  Boar.  Listriodon,  which  had  a  similar  wide 
range,  was  remarkable  in  having  the  cusps  of  the  molar  teeth 
fused  so  as  to  form  complete  transverse  ridges  like  those  of  the 
Tapir.  Elotherium,  which  has  been  found  in  Upper  Eocene  or 
Miocene  strata  both  in  Europe  and  North  America,  was  a  more 
specialized  form.  It  had  two  toes  only  and  the  limbs  were  long 
and  slender.  The  brain  was  remarkably  small.  Hippohyus 
was  a  Lower  Pliocene  pig  found  fossil  in  the  Siwaliks.  It  was 
very  like  Sus  but  had  the  cusps  of  the  molars  modified  in 
irregularly  arranged  laminae. 

Coming  to  the  genus  Sus,  to  which  the  Domestic  Pig  belongs, 
we  find  it  first  in  the  Middle  Miocene  of  France  and  Italy.  This 
species,  which  is  called  S.  choeroides,  had  simple  molars  like 
those  of  the  existing  S.  andamanensis .  Sus  erymanthius,  or 
the  Erymanthian  Boar,  has  been  found  in  the  Red  Crag  of 
Suffolk  and  in  various  continental  countries.  The  still  larger 
S.  titan  and  S.  giganteus  occur  fossil  in  the  Lower  Pliocene  of 
the  Siwalik  Hills1.  There  is  no  evidence,  however,  that  any 
of  these  or  of  the  other  fossil  species  of  Sus  which  have  been 
described  took  part  in  the  formation  of  Sus  scrofa,  which 
includes  the  Wild  Boar  of  Europe,  and  all  the  various  breeds  of 
domesticated  pigs,  excepting  certain  Oriental  varieties  which 
may  have  had  a  different  origin. 

1  See  Smith  Woodward,  Vertebrate  Palaeontology,  Cambridge,  1898. 

21—3 


326  THE   GEOLOGICAL  HISTOEY  [cm. 

Sus  scrofa  itself  occurs  fossil  in  the  Upper  Pliocene  of  Europe. 
In  England  it  has  been  found  in  the  Cromer  Forest  Bed,  and 
it  survived  until  the  seventeenth  century,  when  it  was  still 
abundant  in  Ireland.  Its  remains  have  been  found  also  in 
numerous  Pleistocene  deposits,  such  as  those  of  the  Cambridge- 
shire fens. 

Rutimeyer  and  Nathusius  and  some  other  naturalists  have 
supposed  that  the  various  breeds  of  domestic  pigs  have  been 
descended  from  two  or  more  wild  species,  one  of  which  was 
supposed  to  be  the  same  as  Sus  scrofa  while  another  was  an 
Oriental  species  or  variety  called  by  Pallas  S.  indica.  More 
recently  a  considerable  number  of  other  kinds  of  Oriental  pigs 
have  been  described  (e.g.  Sus  papuensis,  S.  timorensis,  S.  anda- 
manensis,  S.  leucomystax,  etc.),  but  it  is  doubtful  how  far  these 
are  really  distinct.  Moreover  it  has  been  suggested  that  certain 
supposed  species  of  pigs  really  represent  animals  which  have 
become  feral,  and  there  can  be  no  doubt  that  the  wild  or 
semi- wild  pigs  of  certain  islands  in  the  New  World  belong  to 
this  category. 

It  is  probably  safe  to  assume  that  all  the  British  breeds  of 
the  Domestic  Pig  are  the  descendants  of  Sus  scrofa,  which,  as 
we  have  j  ust  seen,  lived  in  Britain  continuously  from  Pliocene 
until  quite  recent  times.  On  the  other  hand  it  is  believed  that 
the  Neoliths  brought  domestic  animals  with  them  from. the 
East,  and  among  these  were  the  Turbary  Sheep,  the  longifrons 
variety  of  the  Ox,  as  well  as  the  Dog  and  the  Pig.  It  is  possible 
therefore  that  the  last  named  animal  may  have  been  in  part 
descended  from  some  wild  species  other  than  Sus  scrofa',  and 
further  that  the  Domesticated  Pig  of  the  Neoliths  may  have 
been  the  part  ancestor  of  some  of  our  modern  breeds.  Such 
suggestions  are  however  extremely  speculative. 

At  the  present  day  the  domesticated  breeds  of  China  and 
Siam  have  broader  and  stouter  heads  than  one  would  be 
inclined  to  expect  in  descendants  of  Sus  scrofa.  It  is  very 
probable  that  these  varieties  are  descended  from  Oriental 
species  other  than  the  Wild  Boar  of  Europe. 

However  this  may  be,  there  are  some  breeds,  such  as  the  old 
Irish  Greyhound  Pigs,  which  are  almost  certainly  the  pure 


xvn]  OF   THE   DOMESTIC   ANIMALS  327 

descendants  of  S.  scrofa,  since  they  resemble  it  in  the  length  of 
their  legs,  the  development  of  their  canine  teeth  into  tusks,  and 
the  comparative  thickness  of  their  hair.  Concerning  the  origin 
of  certain  aberrant  varieties,  such  as  the  solid-hoofed  pigs 
which  were  known  to  Aristotle  and  exist  at  the  present  time  in 
America,  it  is  impossible  even  to  guess. 

THE  DOG 

The  family  Canidae,  to  which  the  Dog,  the  Wolf  and  the  Fox 
belong,  first  appears  in  the  Upper  Eocene  of  Europe,  where  it 
is  represented  by  the  genus  Cynodictis.  This  animal,  which  is 
probably  the  ancestor  of  Canis,  and  possibly  also  of  the  weasel 
family,  was  very  like  the  Dog  in  its  skeletal  characters,  one  of 
the  most  noticeable  differences  being  the  expanded  end  of  the 
humerus.  It  had  five  well-developed  toes  on  each  foot. 

Canine  animals  were  common  in  Miocene  times  in  both 
Europe  and  North  America,  but  the  only  complete  skeletons 
discovered  are  those  of  the  so-called  "Fossil  Fox,"  Galecynus 
oeningensis,  from  the  Upper  Miocene  of  Baden,  and  an  allied 
species,  G.  geismarianus,  from  strata  of  the  same  age  in  Oregon, 
U.S.A.  In  the  European  skeleton  the  first  digit  of  the  forelimb 
is  larger  than  in  the  Dog,  and  in  the  American  skeleton  the 
humerus  resembles  Cynodictis,  to  which  Galecynus  was  obviously 
allied1. 

The  Canidae  appear  to  have  reached  both  India  and  South 
America  in  Pliocene  times.  Subsequently  they  arrived  in 
Australia,  whither  they  were  not  improbably  carried  by  Man. 

The  genus  Canis  first  appears  in  the  Cromer  Forest  Bed, 
where  it  is  represented  by  remains  of  the  Wolf  and  the  Fox. 

The  Domestic  Dog  (Canis  familiaris)  was  probably  the  first 
animal  tamed  by  Man,  but  nothing  is  definitely  known  about 
its  origin.  Its  remains  have  been  found  in  the  Danish  kitchen- 
middens,  and  in  the  Lake  Dwellings  of  Switzerland,  and  various 
deposits  of  contemporaneous  age.  As  already  mentioned, 
there  is  evidence  that  the  Neoliths  in  travelling  westwards 
brought  with  them  a  Domesticated  Dog  from  the  East.  The 

1  See  Smith  Woodward,  loc.  cit. 


328  HISTOKY  OF  DOMESTIC  ANIMALS     [CH.  xvn 

animal  was  on  an  average  about  the  size  of  a  Sheep  Dog.  In 
the  Bronze  Age  a  larger  Dog  existed  and  in  the  Iron  Age  there 
was  a  still  larger  variety.  The  Jackal  (of  which  there  are  several 
species),  the  Indian  Wolf  (C.  pallipes),  and  the  Bunasu  (C.  pri- 
maevus)  have  all  been  suggested  as  ancestors  of  the  Dog.  It  is 
possible  that  there  were  really  several  ancestors  which  are 
represented  in  various  proportions  in  the  two  hundred  or  more 
varieties  which  exist  at  the  present  day.  In  the  absence  of 
more  evidence  a  further  discussion  of  the  problem  as  to  the 
origin  of  the  Domesticated  Dog  would  be  unprofitable.  It  may 
be  pointed  out,  however,  that  physiologically  there  is  no 
specific  distinction  to  be  drawn  between  the  Dog  and  the  Wolf 
and  certain  other  wild  species  of  Canis,  since  these  animals  will 
generally  breed  together  in  confinement  and  produce  offspring 
which  are  fertile. 


INDEX 


Albite,  6 

Alkali  soils,  158 

Alluvium,  62,  67,  124,  127 

Amphibole,  55 

Ampthill  Clay,  265 

Andesite,  34 

Anhydrite,  44,  106 

Anorthite,  6 

Anticline,  21 

Apatite,  9,  100 

Arid  regions,  49,  72,  241 

Arkose,  84 

Artesian  wells,  173 

Augite,  8 

Bacteria,  39,  54,  142,  144 

Bajocian  series,  257 

Bala  series,  206 

Basalt,  34,  287 

Basin,  21 

Bathonian  series,  257 

Beaches,  83 

Bedding,  16 

Biotite,  7,  55 

Black  earth,  88,  136,  148 

Blackheath  beds,  285 

Borrowdale  Volcanic  series, 

Boss,  14 

Boulder,  75,  81,  115,  292 

Boulder-clay,  75,   113,  130 

Brachiopods,  205,  209,  210 

Breccia,  27,  82 

Brockram,  83 

Brown  soils,  154,  165,  181 

Bunter  series,  244 

Bysmalith,  14 

Calcite,  9 

Calcium  carbonate,  43,  81 

Cambrian  system,  205 

Cambridge  Greensand,  102 

Canon,  70 

Carbonation,  47 

Carboniferous  system,  226 

Carnallite,  106 

Caves,  71,  77 

Cement,  81,  113 


207 


Chalk,  71,  93,  99,  103,  113,  126,  131. 

148,  172,  177,  278,  294 
Chalk-marl,  93,  278 
Chalky  boulder-clay,  298 
China-clay,  52,  89,  219 
Chert,  99' 
Clay,  57,  87,  133 
Clay-with-flints,  114,  281 
Cleavage,  16,  20,  36 
Cliffs,  77 

Climate,  39,  193,  199,  213 
Climatic  zones,  40,  60,  145,  162 
Coal-measures,  234 
Coast-erosion,  76 
Colloidal  silica,  52 
Colloids,  52,  87 
Columnar  structure,  18 
Conglomerate,  82,  113 
Contour  lines,  183 
Coombe  rock,  124,  296 
Coprolites,  102 
Corallian  series,  264 
Coral  rock,  92,  101,  265 
Cornbrash,  258 
Cornstone,  90,  220 
Corrasion,  60 
Crag,  288 

Craven  faults,  229,  233 
Cretaceous  system,  269 
Cromer  Forest  bed,  289 
Culm,  226,  239 

Dalradian  series,  202 

Denudation,  63 

Dew-ponds,  176 

Diorite,  33 

Dip,  17,  24,  63,  188 

Divisional  planes,  15 

Dog,  327 

Dolerite,  34 

Dolomite,  9,  43,  93,  154,  24 

Dolomitic  Conglomerate,  245 

Dome,  21 

Downs,  72,  177,  281 

Drainage,  173 

Drift,  131,  291 

Dunes,  134 


330 


INDEX 


Dykes,  13,  170 

Earth-movements,  216,  220,  240,  283, 

287 
Eocene  series,  284 

Facies,  199,  216 

Fault- breccia,  27 

Faults,  16,  23,  188 

Felspar,  6,  32,  44,  52 

Fenland,  127,  173 

Fire-clay,  89 

Flags,  85 

Flint,  98 

Folds,  21 

Foliation,  16,  20,  36 

Forest  soils,  150 

Fossil-zones,  208 

Fox-bench,  143 

Frost,  50 

Fullers'  earth,  89,  258 

Gabbro,  33,  287 

Garnet,  10 

Gault,  275 

Glacial  deposits,  129,  291 

Glaciers,  73,  213 

Glass,  29,  31 

Glauconite,  85,  101,  103 

Gneiss,  21,  36,  200 

Granite,  33,  115,  148,  202,  218,  222, 

287 

Graptolites,  206,  208,  210 
Gravels,  125,  130 
Great  Oolite,  257 
Greisen,  54 
Grit,  84 

Guano,  101,  108 
Gypsum,  9,  44,  58,  84,  87,  106,  246 

Harlech  grits,  205 

Hastings  Sands,  270 

Hertfordshire  Pudding  Stone,  83,  113, 

285 

Hornblende,  8,  56 
Horse,  302 
Hydration,  43 
Hydrolysis,  44,  52 

Ice-erosion,  69,  73 

Igneous  rocks,  2,  11,  18,  29,  30,  286 

Indian  soils,  159 

Inferior  Oolite,  257 

Iron  pyrites,  8,  47,  58 

Ironstone,  95,  254,  265 

Joints,  16,  71 
Jurassic  system,  251 

Kainite,  106 


Kankar,  141,  153,  160 
Kaolin,  35,  219 
Kaolinite,  45,  52,  57 
Karst,  71,  233 
Kellaways  Rock,  263 
Keuper  series,  244 
Killas,  218 
Kimeridge  Clay,  266 
Kimeridgian  series,  263 

Laccolith,  14 

Lamination,   16 

Laterite,  54,  57,  115,  153,  161 

Lava,  12,  30,  206,  222 

Lewisian    gneiss,  201 

Lias,  253 

Limestone,  70,  90,  115,  118,  154 

Limestone,  magnesian,  93 

Lingula  Flags,  205 

Lit-par-lit  injection,  15 

Llanberis  slates,  205 

Llandeilo  series,  206 

Llandovery  series,  209 

Loam,  144 

Loess,  88,  136,  139,  147,  148,  152 

London  Clay,  285 

Lower  Greensand,  272 

Magma,  31 

Magnesian  limestone,  241 
Magnetite,  8 
Maps,  182,  195 
Marl,  87,  93,  107,  144 
Marlstone,  255 
Meanders,  67 
Metamorphic  rocks,  3,  35 
Mica,  7,  53,  55,  85 
Millstone  Grit,  231 
Minerals,   1,  31 
Mineral  springs,  46 
Mineral  veins,  27 
Miocene  series,  287 
Mixed  crystals,  4 
Moine  gneisses,  202 
Moraines,  73,  133,  152 
Mud,  81,  87 
Muscovite,  7,  55 

Neck,  14 
Nitrates,  108 

Old  Red  Sandstone,  199,  216,  220 

Oligocene  series,  284 

Olivine,  8,  56 

Oolite,  91,  251 

Orthoclase,  6,  32 

Ortstein,  140,  149 

Outcrop,  17,  190 

Overlap,  27 

Overstep,  27 


INDEX 


331 


Ox,  312 

Oxford  Clay,  263 
Oxidation,  46,  141 
Oxides,  3 

Pans  in  soils,  96,  118,  140,  175 
Peat,  118,  128,  203,  213 
Pegmatite,  29 
Pendleside  group,  232 
Peridotite,  33 
Permian  system,  241 
Phosphates,  35,  100,  103,  280 
Phosphoric  acid,  10,  85,  97 
Pig,  324 
Pisolite,  91 
Plagioclase,  6,  32,  54 
Pleistocene  series,  291 
Pliocene  series,  288 
Ploughing,  50 
Plutonic  rocks,  32 
Pneumatolysis,  45,  53,  219 
Podzol,  150,  165,  181 
Ponds,  175 
Porphyrite,  34 
Porphyritic  texture,  29 
Porphyry,  34 
Portland  series,  263,  267 
Pot-holes,  69 
Precambrian  rocks,  201 
Purbeckian  series,  263,  267 
Pyroxene,  55 

Quartz,  5,  32,  51,  86 
Quartzite,  36,  117 
Quartz- porphyry,  34 

Rainfall,  72,  162,  166,   177 

Recent  formations,  291 

Red  soils,  154,  161 

Reduction,  47 

Rhaetic  series,  244 

Rhyolite,  34 

Rivers,  63 

Rock-forming  minerals,  4 

Rock-salt,  9,  58,  87,  246 

Rocks,  chemical  composition  of,  3 

—  classification  of,  10 

—  definition  of,  1 

—  divisional  planes  in,  15 
Rock-structures,  11 
Rock-systems,  197 
Rock-textures,  12,  28 

Saline  soils,  151,  154 

Salt  deposits,  104 

Sand,  72,  81,  83,  131,  134 

Sand-hills,  134 

Sandstone,  83 

Sarsen  stones,  113,  285 

.Schist,  21,  35,  200 


Schistosity,  16,  20 

Screes,  123 

Sedimentary  rocks,  2,  11,  28 

Serpentine,  45,  56 

Shale,  87 

Sheep,  320 

Sheet,  13 

Silica,  52,  81 

Sill,  13 

Silurian  system,  209 

Slaking  of  lime,  43 

Soils,  34,  38,  62,  137,  143 

Soil-zones,  146,  163 

Speeton  Clay,  269,  275 

Springs,  52,  169 

Stratification,   16 

Strike,  17 

Subsoil,  137 

Superficial  deposits,  111 

Swallo whole,  71 

Swamp  deposits,  122,  151 

Syenite,  33 

Syncline,  21 

Terraces,  125 
Terra  rossa,  154 
Tertiary  systems,  283 
Thanet  sands,  285 
Thrust-plane,  25 
Torridon  Sandstone,  201 
Trachyte,  34 
Transport,  60 
Tremadoc  slates,  205 
Triassic  system,  244 
Trilobites,  205,  208,  210 
Tundra,  120,  151 

Unconformity,  23,  27,  222,  240 
Underground  water,  167 
Upper  Greensand,  275 
Uralitization,  56 

Veins,  27,  219 
Volcanic  rocks,  30,  33 

Warp,  129 

Waterfalls,  69 

Water-stones,  246 

Water  supply,  166 

Water-table,  168,  174 

Weald  Clay,  270 

Weathering,  38,  48,  137 

Wells,  171 

Wenlock  series,  209 

Wind,  51,  134,  178,  297 

Woolwich  and  Reading  beds,  285 

Yoredale  series,  229 
Zeolites,  44 


Cambridge : 

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